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235 feet above us, the sea was smooth and clear of ships, the atmosphere was without a cloud, and there was no object in sight which could possibly produce the observed effect. From the perfectly transparent air the echoes came, at first with a strength apparenUy but Utile less than that of the direct sound, and then dying gradually and continuously away. The remark of my companion, Mr. Edwards, was: "Beyond saying that the echoes seemed to come from the expanse of ocean, it did not appear possible to indicate any more definite point of reflection." Indeed, no such point was to be seen; the echoes reached us as if by magic, from absolutely invisible walls. Arago's notion that clouds are necessary to produce atmospheric echoes is therefore untenable.
The reflection fiom aerial surfaces has never been experimentally demonstrated. It is wholly a matter of inference, and I wished very much lo reduce it to demonstration. I made one or two rough experiments on the transmission of sound through a series ol flames; and no doubt by proper arrangement such experiments might be made successful. I then thought that alternate layers of carbonic acid and coal £as, the one rising by its lightness, the other falling by its weight, would supply a heterogeneous medium suitable for the demonstration. To my assistant, Mr. Cottrell, who possesses in an eminent degree the skill of devising appara'us, I communicated this idea, leaving the realisation of it wholly to him, and he has carried it out in the most admirable manner. (For a sketch and description of the apparatus, see previous page).
During my recent visit to the United States I accompanied General Woodruff, the engineer in charge of two of the lighthouse districts, to the establishments at Staten Island and Sandy Hook, with the express intention of observing the performance of the steam-syren, which, under the auspices of Prof. Henry, has been introduced into the lighthouse system of the United States. Such experiments as were possible to make under the circumstances were made, and I carried home with me a somewhat vivid remembrance of the mechanical effect of the sound of the steam syren upon my ears and body generally. This I considered to be greater than the similar effect produced by the horns of Mr. Holmes ; hence the desire, on my part, to see the syren tried at the South Foreland. The formal expression of this desire was anticipated by the Elder Brethren, while their wishes were in turn anticipated by the courteous kindness of the Lighthouse Board at Washington. Informed by Major Elliott that our experiments had begun, the Board forwarded to the Corporation, for trial, the noble instrument now mounted at the South Foreland. The principle of the syren is easily understood. A musical sound is produced when the tympanic membrane is struck periodically with sufficient rapidity. The production of these tympanic shocks by puffs of air was first realised by Dr. Robison. But the syren itself is the invention of Cagniard de la Tour. He employed a box with a perforated lid, and above the lid a similarly perlora'ed disc, capable of rotation. The perforations were oblique, so that when wind was driven through the disc was set in motion. When the perforations coincided a puff escaped, when they did not coincide the current of air was cut off. The regular succession of impulses thus imparted to the air produce a musical note. Even in its small form, the instrument is capable of producing sounds of great intensity. The syren has been improved upon by Dove, and notably developed by llelmholtz.
In the steam syren patented by Mr. Brown of New York, a fixed disc and a rotatory disc are also employed, radial slits being cut in both discs instead of circular apertures. One disc is fixed across the throat of a trumpet-shaped tube, i6J ft. long, 5"in. diameter where the disc crosses it, and gradually opening out till at the other extremity it reaches a diameter of 2 ft. 3 in. Behind the fixed disc is the rotating one, which is driven by separate mechanism. The trumpet is mounted on a boiler. In our experiments steam of "o lbs. pressure has for the most part been employed. Just as in the air-syren, when the radial slits of the two discs coincide, a puff of steam escapes. Sound-waves of great intensity are thus sent through the air; the pitch of the note produced depending on the rapidity with which the puffs succeed each other; in other words, upon the velocity of rotation.
On October 8 I remained some lime at the Foreland, listening to the echoes. Of the horn-echoes I have already spoken : those of the syren were still more extraordinary. Like the others they were perfectly continuous, and faded as if into the distance gra
dually away. The single sound seemed rendered complex and multitudinous by its echoes, which resembled a band of trumpeters first responding close at hand, and then retreating ratiidly from us towards the coast of France. The syren echoes had eleven seconds duration, those of the horn eight seconds. With sorisdi of the same pitch the duration of the echo might be taken ts 4 measure of the penetrative power of the sound.
I moved away from the station so as to lower the power of the direct sound. This was done by dropping into the soundshadow behind an adjacent eminence. The echoes heard thus were still more wonderful than before. In the case of the syren, moreover, the reinforcement of the direct sound by the echo was distinct. One second after the commencement of the syren lilast, the echo struck in as a new sound. This fir.>t echo, therefore. must have been flung back by a body of air not more than 600 or 700 ft. in thickness.
There appears to be a direct connection between the duration of the echoes and the distance penetrated by the sound. Oa October 17 the perfect clearness of the afternoon caused me to choose it for the examination of the echoes. The echoes of that day, when our transmitted sound reached its maximum, exceeded in duration those of all other days. We heard the syren fifteen miles off. On the close of the day we found its echoes fourteen to fifteen seconds in duration, this long duration indicating the distance from which they were thrown back.
The visual clearness of the atmosphere on the morning of Oct. S, wasverygreat, the coast of France was very plainlysern, iheGrisna lighthouse, and the monument and cathedral of Jioulogne, were distinctly visible to the naked eye. At j) miles from the station, the horn was heard feebly, the syren clearly. At 2.30 P.m., .1 densely black scowl overspread the heavens to the W.S.W. At this hour, the distance being 6 miles, the horn wis heard very feebly, the syren more distinctly, all being hushed on board during the observations. A squall now approached us from the west. In the Alps, or elsewhere, I have rarely seen the beirens blacker. Vast cumuli floated in the X.E. and S.E.; vast streamers of rain were seen descending W.N. W. ; huge scrolls of cloud to the N.
At 7 miles distance the syren was not strong, and the hom was very feeble.
The heavy rain at length reached us, but although it was falling all the way between us and the Foreland, the sound, instead of being deadened, rose perceptibly in power. Hail was now added to the rain, and the shower reached a tropical violence. We stopped. In the midst of this furious squall both the hom and ihe syren were distinctly heard, and as the shower lightened, thus lessening the local pattering, the sounds so rose in power that we heard them at a distance of 7 J miles distinctly louder than they had been heard through the rainless atmosphere at 5 miles. This observation is entirely opposed to the statement of Derham, which has heen repeated by all writers since his time, regarding the stifling influence of falling rain upon sound. Hut it harmonises perfectly with our experience of July 3, which proved water in a state of vapour so mixed wiih air as to form non-homogeneous parcels, to be a most potent influence as regards the stoppage of sound. Prior to the violent shower, the nir had been in this flocculcnt condition, but the descent of the rain and hail restored in part the homogeneity of the atmosphere, and augmented its transmissive power. There may be states of the atmosphere associated with rain unfavourable to sound, but to rain itself I have never been able to trace the slightest deadening effect.
The observations continued till November 25. Up to that date we had no fog, but the experience of July I and of October 30, entirely destroy the notion that optical transparency and acoustic transparency go hand-in-hand. Both were days of ha:e sufficiently thick to hide the cliffs of the Foreland, but on the former the sounds reached 12 J. and on the latter 11J miles.
Keflection from the particles of fog and ha/.e has been hitherto held to blot out sound. The late dense fog in London enabled experiments to be made which entirely controvert this conclusion. On December 10 I made some experiments over the Serpentine. The fog was verydense. Mr. Cotterell stood on the walk below the south-west end of the bridge dividing Hyde Park from Kensington Gardens, while I went to the eastern end of the Serpentine. He blew a dog-whistle, and an organ-pipe sounding Mi3, which corresponds to 380 waves a second. I heard both distinctly. I then changed places with him, and listening attentively at the 1 ridge, heard for a time the distinct blast of the whistle only. The organ-pipe at length sent its deeper note to me across the water. It sometimes rose to great '■ istinctness, and sometimes fell to inaudibility. These fluctuations of which various striking examples have been observed, are due to the drifting of acoustic clouds, which act upon a source of sound, as the drifting of ordinary clouds upon the sun. The whistle showed the same intermittence as to period, but in the opposite sense for when the whistle was faint the pipe was strong, and vice versd.
There seemed to be an extraordinary amount of sound in the air. It was filled with a resonant roar from the Bayswater and Knightsbridge roads. The railway whistles were extremely distinct, while the fog-signals exploded at the various metropolitan stations kept up a loud and almost constant cannonade. I could by no means reconcile this state of things with the statements so categorically made regarding the influence of fog.
The water was on this day warmer than the air, and the ascending vapour was instantly in part condensed, thus revealing its distribution. Instead of being uniformly diffused, it formed wreaths and stria?. I am pretty confident that had the vapour been able to maintain itsell as such, the air would have been far more opaque to sound. In other words, I believe that the very cause which diminished the optical transparency of the atmosphere augmented its acoustical transparency.
This conclusion was confirmed by numerous observations made while the fog lasted.*
On Dec. 13 the fog was displaced by a thin haze. We could plainly see from one bank of the S .rpentine to the other, and far into Hyde Park beyond. There was a wonderful subsidence of the sound of the carriages, church bells, &c. Being at the bridge I listened for the sounds excited at the end of the Serpentine. With the utmost stretch of attention I could hear nothing. I walked along the edge of the water towards Mr. Cottrcll, and when I had lessened the distance by one half, the sound of his whistle was not so distinct as it had been at the bridge on the day of the densest fog. Hence the optical cleansing of the air by the melting of the fog had so darkened it acoustically, that a sound generated at the end of the Serpentine was lowered to at least one-fourth of its intensity at a point midway between the end and the bridge.
This opportune fog enabled me to remove the last of a congeries of errors which, ever since the year 1708, have attached themsdves to this question. As regards phonic coast-jignals, we now know exactly where we stand.
It is worth observing he e that the solution of the deportment of bail, rain, snow, haze, and log, as regards sound, der ends entirely upon observations made on the 3rd of July, which was about the last day that one would have chosen lor experiments on fog-signali Indeed, it had been distinctly laid down that observations on such a day would be useless; that they might indeed cnab'e us to weed away bad instruments from good ones, but could throw no light whatever on the quest in of fogsignaling. That the contrary is the case, is an illus'ration of the fact that the solution of a question often lies in a direction diametrically opposed to that in which it appears to lie. t
EXTRACTS FROM AN ADDRESS BY STR IV. THOMSON, TO THE SOCIETY OF TELEGRAPHIC ENGINEERS
T HAVE advisedly, not thoughtlessly, used the expression ■*■ "terrestrial electricity." It is not an expression we are accustomed to. We are accustomed to "terrestrial magnetism;" we are accustomed to atmospheric electricity. The electric telegraph forces us to combine our ideas with reference to terrestrial magnetism and atmospheric electricity. We must link upon the earth and the air as a whole—a globe of eirlh and air—and consider its electricity whether in test or in motion. Then, as to terrestrial magnetism, of what its relation may be to perceptible electric manifestations we at present know nothing.
You all know that the earth acts as a great magnet. Dr. Ciilbert, of Colchester, made that clear nearly 300 years ago; but how the earth acts as a great magnet—how it is a magnet,— whether an electro magnet in \irtue of currents revolving round under the upper surface, or whether it is a magnet like a mass of steel or load-stone, we do not know. This we do know, that
* Since the first notices of this lecture appeared in the newspapers, strong confirmatory evidence has been received.
t The 'Oregoing report vat compiled from the notes of Prof. Tyndall. It if published with Prof. Tyndall's sanction, but was not written by himvelf.
it is a variable magnet, and that a first approximation to the variation consists in a statement of motion round the axis of figure—motion of the magnetic poles, round the axis of figure, in a period of from 900 to 1,000 years. The earth is not a uniformly magnetised magnet with two ]>olcs. and with circles of symmetry round those poles. But a first expression—as we should say in mathematical language the first "harmonic term" —in the full expression of terrestrial magnetism is an expression of a regular and symmetrical distribution such as I have indicated. Now, this is quite certain, that the axis of this first term, so to speak, or this first approximation, which, in fact, we might call the magnetic axis of the earth, does revolve round the axis of figure.
When the phenomena of terrestrial magnetism were first somewhat accurately observed about three hundred years ago, the needle pointed here in England a little to the east of north; a few years later it pointed due north; then, until about the year 1820, it went to the west of north ; and now it has come back towards the north. The dip has experienced corresponding variations. The dip was first discovered by the instrument maker, Robert Norman, an illustration, I may mention in passing, of the benefits which abstract science derives from practical applications—one of the most important fundamental discoveries of magnetism brought back to theory by an instrument maker who made mariner's compasses. Robert Norman, in balancing his compass cards, noticed that after they were magnetised one end dipped, and he examined the phemonenon and supported a needle about the centre of gravity, magnetised it, and discovered the dip. When the dip was first so discovered by Robert Norman it was less than it is now. The dip has gone on increasing, and is still increasing ; but abcut 50 years ago the deviation from true north was greatest. Everything goes on as if the earth had a magnetic pole revolving from west to east round the true North Pole, at a distance of 200 fiom it. About three hundred years ago its azimuth from V ngland was a little to the east of the north pole: then it came round, moving eastwards on the far side of the north pole, and round in a circle towards us on the left-hand side of the north pole, as looked to from England. That motion in a circle round the north pole has already been experienced within the period during which somewhat accurate measurements have been made—has been experienced to the extent of rather more than a quarter of the whole revolution; and we may expect that about 200 years from the present lime the magnetic pole will be between England and the North Pole; so that the needle will thus point due north, and the dip be greater than it has been for I,coo years, or will be for another. It is one of the greatest mysteries o' science, a mystery which 1 might almost say is to myself a subject of daily contemplation— what can be the cause of this magnetism in the interior of the earth? Rigid magnetisation, like that of steel or the loadstone, has no quality in itself in virtue of which we can cjneeive it to migrate round in the magnetised bar. Electric currents afford the more favoured hypothesis; they are more mobile. If we can conceive electric currents at all, we may conceive them flitting about. But what sustains the electric currents? People sometimes say, heedlessly or ignorantly, that thermoelectricity does it. Wc have none of the elements of the problem of thermo-electricity in the state of underground temperature which could possibly explain, in accordance with any knowledge we have of thermo-electricity, how there could so be sustained currents round the earth. And if there were currents round the earth, regulated by some cause so as to give them a definite direction at one time, we are as far as ever from explaining how the channel of those currents could experience that great secular variation which we know it does. Thus we have merely a mystery. It would be rash to suggest even an explanition. I may say that one explanation has been suggested. It was suggested by the great astronomer, Halley, that there is a nucleus in the interior of the earth, and that the mystery is explained simply by a magnet not rigi ily connected with the upper crust of the earth, but revolving round an axis differing from the axi? of roation of the outer crust, and exhibiting agradual precrssional motion independent of the precessional motion of the outer rigid crust. I merely say that has been suggested. I do not ask }ou to judge of the pr ibability: I would not ask myself to judge of the probability of it. No other explanation has been suggested.
But now, I say, we loo'* with hopefulness to the practical telegraphist for data towards a solu ion of this grand p-obl-n.. The terrestri d m gnet is subject, as a whole, to the grand secular variation which I have indicated. But, besides that, there are annual variations and diurnal variations. Every day the needle varies from a few minu'es on one side to a few minutes on the other side of its mean position, and at times there are much greater variations. What are called "magnetic storms" are of not very unfrequent occurrence. In a magnetic storm the needle will often fly twenty minutes, thirty minutes, a degree, or even as much as two or three degrees sometimes, from its proper position—if I may use that term—its proper position lor the time; that is, the position which it might be expected to have at the time according to the statistics of previous observations. I speak of the needle in general. The ordinary observation of the horizontal needle shows these phenomena. So does observation on the dip of the needle. So does observation on the total intensity of the terrestrial magnetic force.
The three elements, deflection, dip, and total intensity, all vary every day with the ordinary diurnal variation, and irregularly with the magnetic storm. The magnetic storm is always associated with a visible phenomenon, which we call, habitually, electrical;—aurora borealis, and, no doubt, also aurora of the southern polar regions. We have the strongest possible reasons for believing tha.t aurora consists of electric currents, like the electric phenomena presented by currents of electricity through what are called vacuum tubes, through the space occupied by vacuums of different qualities in the well-known vacuum tubes. Of course, the very expression, "vacuums of different qualities" is a contradiction in terms. It implies that there are small quantities of matter of different kinds left in those nearest approaches to a perfect vacuum which we can make.
Well now, it is known to you all that aurora borealis is properly comparable with the phenomena presented by vacuum tubes. The appearance of the light, the variations which it presents, and the magnetic accompaniments, are all confirmatory of this view, so that we may accept it as one of the truths of science. Well now—and htre is a point upon which, I think, the practical telegraphist not only can, but will, before long give to abstract science data forjudging—is the deflection of the needle a direct effect of the auroral current, or are the auroral current and the deflection of the needle common results of another cause?
With reference to this point, I must speak of underground currents. There again I have named a household word to everyone who has anything to do with the operation of working the electric telegraph, and not a very pleasing household word I must say. I am sure most practical telegraphers would rather never hear of earth currents again. Still we have got earth currents; let us make the best of them. They are always with us; let us see whether we cannot make something of them, since they have given us so much trouble. Now, if we could have simultaneous observations of the underground currents, of the three magnetic elements, and of the aurora, we should have a mass of evidence from which, I believe, without fail, we ought to be able to conclude an answer more or less definite to the question I have put. Are we to look in the regions external to our atmosphere for the cause of the underground currents, or are we to look under the earth for some unknown cause affecting terrestrial magnetism, and giving rise to an induction of those currents? The direction of the effects, if we can only observe those directions, will help us most materially to judge as to what answer should be given.
It is my desire to make a suggestion which may reach members of this society, ami associates in distant parts of the world. 1 make it not merely to occupy a little time in an inaugural address, but with the most earnest desire and expectation that something may be done in the direction of my suggestion. I do not venture to say that something may come from my suggestion, because, perhaps, without any suggestion from me, the acute and intelligent operators whom our great submarine telegraph companies have spread far and wide over the earth, are fully alive to the importance of such observations as I am now speaking of. I would just briefly say that this kind of observation is what ■would be of value for the scientific problem—to observe the indication of an electrometer at each end of a telegraph line at any time, whether during a magnetic storm or not, and at any time of the night or day. If the line be worked with a condenser at each end, this observation can be made without in the slightest degree influencing, and therefore without in the slightest degree disturbing, the practical work throughout the line. Put on an electrometer in direct connection with the line, connect the outside of the electrometer with a proper earth connection, and it may be observed quite irrespectively of the signalling; when the
signalling is done, as it very frequently is at submarine line*, with a condenser at each end. The saentiric observation mil be disturbed undoubtedly, and considerably distorted by the sending of messages, but the disturbance is only transient, and in the very pause at the end of a wor.i there will !>e a surheiently near approach to steadiness in the potential at the end of a «t>: connected with the electrometer to allow a careful observer to estimate with practical accuracy the indication that he would tar; were there no working of the line going on at the lime. A rnignetic storm of considerable intensity does not stop the working, does indeed scarcely interfere with the working, of a submarine line in many instances when a con lenser is used at each end.
Thus, observations, even when the line is working, may be made during magnetic storms, and again, during hours when the line is not working, if there are any, and even the very basic*: lines have occasional hours of rest. Perhaps, then, however, the operators have no time or zeal left, or, rather, 1 am sure they have always zeal, but I am not sure that theie is always time left, and it may be impossible for them to bear the strain longer than their office hours require them. But when there is an operator, or a superintendent, or a mechanic, or an extra operator who may have a little time on his hands, then, I say, any single observation or any series of observations that he can make on the electric potentials at one end of an insulated line will give valuable re uIts. When arrangements can be made for simultaneous observations of the potentials by an electrometer at the two ends of the line, the results will be still more valuable.
And, lastly, I may just say that when an electrometer is not available, a galvanometer of very large resistance may be employed. This will not in the slightest decree interfere with the practical working any more than would an electrometer, nor will it be more difficult to get results of the scientific observations not overpoweringly disturbed by the practical working if a galvanometer is used than when an electrometer is available. Tie more resistance that can be put in between the cable ani the earth in circuit with the galvanometer the better, and the sensibility of the galvanometer wilt still be found perhaps more than necessary. Then, instead of reducing it by a shunt, let steel magnets, giving a more powerlul direction to the needle, be applied for adjusting it. The resistance in circuit with the galvanometer between cable-end and earth ought tu be at least twenty-times the cable's copper-resistance to make the galvanometer observations as valuable as those to be had by electrometer.
I should speak also of the subject of atmospheric electricity. The electric telegraph brings this phenomenon into connection with terrestrial magnetism with earth current-, and through them with aurora borealis, in a manner for which observations made before the time of the electric telegraph, or without the aid of the electric telegraph, had aot given us any data whatever. Scientific observations on terrestrial magnetism, and on the aurora, and on atmospheric electricity, had shown a connection between the aurora and terrestrial magnetism in the shape of the disturbances that I have alluded to at the time of magnetic storms; but no connection between magnetic storms and atmospheric electricity, thunderstorms, or generally the state of the weather—what is commonly called meteorology—has vet been discovered.
The one common link connecting these phenomena hitherto known to us is exhibited in the electric telegraph. A telegraphic line—an air line more particularly, but a submarine line also— shows us unusually great disturbances, not only when there are auroras and variations of terrestrial magnetism, but when the atmospheric electricity is in a disturbed state. That it should be so electricians here present will readily understand. They will understand when they consider the change of electrification of the earth's surface which a lightning discharge necessarily produces.
1 fear I might occupy too much of your time, or else I would just like to say a word upon atmospheric electricity, and to call your attention to the quantitative relations which questions 10 connection with this subject bear to those of ordinary earth currents and the phenomena of terrestrial magnetism. In fair weather, the surface of the earth is always, in these countries it all events, found negatively electrified. Now the limitation to these countries that I have made suggests a point for the practical telegraphists all over the world. Let us know whether it is only in England, France, and Italy that in fine weather the earth's surface is negatively electrified.
The only case of exception on record to this statement is Prff. Plaza Smvth's observations on the Peak of Teneriffe. There, during several months of perfectly fair weather, the surface of the mountain was, if the electric test applied was correct, positively electrified; but Prof. 1'ia^zi Smyth has, I believe, pointed out that the observations must not be relied upon. The instrument, as he himself found, was not satisfactory. The science of observing the atmospheric electricity was then so much in its infancy tha% though he went prepared with the best instrument, and the only existing rules for usin>; it, there was a fatal doubt as to whether the electricity was positive or negative after all._ But the fact that there has been such a doubt is important. Now I suppose there «ill be a telegraph to Teneriffe before long, and then I hope and trust some of the operators will find time to climb the Peak. I am sure that, even without an electric object, they will go up the Peak. Now they must go up the Peak with an electrometer in fine weather, and ascertain whether the earth is positively or negatively electrified. If they find that on one fine day it is negatively electrified, the result will be valuable to science; and if on several days it is found to be all day and all ni^ht negatively electrified, then there will be a very great accession to our knowledge regarding atmospheric electricity.
When I say the surface of the earth is negatively electrified, I make a statement which I believe was due originally to Peltier. The more common form of statement is that the air is positively electrified, but this form of statement is apt to be delusive. More than that, it is most delusive in many published treatises, both in books and encyclopedias upon the subject. 1 have in my mind one encyclopedia in which, in the article "Air, Klectricity of," it is said that the electricity of the air is positive, and increases in rising from the ground. In the same encyclop.-edia, in the article "Electricity, Atmospheric," it is stated that the surface of the earth is negatively electrified, and that the air in contact with the earth, and for some height above the earth, is, in general, negatively electrified. I do not say too much, then, when I say that the statement that the air i< positively electrified has been at all events a subject for ambiguous and contradictory proposition-;; in fact, what we know by direct observation is, that the surface of the earth is negatively electrified, and positive electrification of the air is merely inferential.
Suppose, for a moment, that there were no electricity whatever in the air—that the air were absolutely devoid of all electric manifestation, and that a charge of electricity were given to the whole earth. For this no great amount would be necessary. Such amounts as you deal with in your great submarine cables would, il given to the earth as a whole, produce a very considerable electrification of its whole surface. You all know the comparison between the electricity of one Atlantic cable—the electro-static capacity of one of the Atlantic cables—with the water round its guttapercha for outer coating, and the earth and air with infinite space for its outer coating.* I do not remember the figures at this moment; in fact, I do not remember which is the greater. Well, now, if all space were non-conducting—and experiments on vacuum tubes seem rit'ner to support the possibility of that being the correct view—if all space were non-conducting, our atmosphere being a Doa-conductor, and the rarer and rarer air above us being a non-conductor, and the so called vacuous space, or the interplanetary space beyond that (which wc cannot admit to b; really vacuous) being a non-conductor also. then a charge could be given to the earth as a whole, if there were the other body to come and go oway again, just as a charge could be given to a pith ball electrified in the air of this room. Then, I say, all the phenomena brought to light by atmospheric electrometers, which wc observe on a fine day, would be observed just as they are. The ordinary observation of atmospheric electricity would give just the result that we obtain from it. The result that we obtain every day in observations on atmospheric electricity is precisely the same as if the earth were electrified negatively and the air had no electricity in it whatever.
Well, now I have asserted strongly that the lower regions of the air are ncgitively electrified. On what foundation is this assert on made? Simply by observation. It is a matter of fact; it t' not a matter of speculation. 1 find that when air is drawn into a room from the outside, on a fine day, it is negatively e'.cctrified. I believe the same phenomena will be observed in this city as in the old buildings of the Uni
* The eajlh'sradius is about 630 million centimetres, and its electrostatic capacity is therefore 630 microfarads, or about that of 1,600 miles of cable.
versity of Glasgow, in the middle of a very denselypeopled and smoky part of Glasgow; and therefore I doubt not that when air is drawn into this room from the outside, and a water-dropping collector is placed in the centre of the room, or a few feet above the floor, and put in connection with a sufficiently delicate electrometer, it will indicate negative electrification. Take an electric machine; place a spirit lamp on its prime conductor; turn the machine for a time ; take an umbrella, and agitate the air with it till the whole is well mixed up; and keep turning the machine, with the spirit-lamp burning on its prime conductor. Then apply your electric test, and you find the air positively electrified.
Again—Let two rooms, with a door and passige between them, be used for the experiment. First shut the door and open the windoA" in your observing room. Then, whatever electric observations you may have been performing, after a short time you find indications of negative electrification of the air. Then, during all that time, let us suppose that an electric machine has been turned in the neighbouring room, and a spirit-lamp burning on its prime conductor. Keep turning the electric machine in the neighbouring room, with the spirit-lamp as before. Make no other difference but this—shut the window and open the door. I am supposing that there is a fire in your experimenting room. Then, when the window was open and the door closed, the fire drew its air from the window, and you got the air direct from without. Now shut the window and open the door into the next room, and gradually the electric manifestation changes. And here somebody may suggest that it is changed because of the opening of the door and the inductive effect from the pissage. But I anticipate that criticism by saying that my observation has told me that the change takes place gradually. For a time after the door is opened and the window closed, the electrification of the air in your experimenting room continues negative, but it gradually becomes zero, and a little later becomes positive. It remains positive as long as you keep turning the electric machine in the other room and the door is open. If you stop turning the electric machine, then, after a considerable time, the manifestation changes once more to the negative; or if you shut the door and open the window the manifestation changes more rapidly to negative. • It is, then, proved beyond all doubt that the electricity which comes in at the windows of an ordinary room in town is ordinarily negative in fair weather. It is not always negative, however. I have found it positive on some days. In broken weather, rainy weather, and so on, it is sometimes positive and sometimes negative. Now, hitherto, there is no proof of positive electricity in the air at all in fine weather; but we have grounds for inferring that probably there is positive electricity in the upper regions of the air. To answer that question the direct manner is to go up in a balloon, but that takes us beyond telegraphic regions, and therefore I must say nothing on that point. But I do say that superintendents and telegraph operators in various stations might sometimes make observations; and I do hope that the companies will so arrange their work, and provide such means for their spending their spare time, that each telegraph-station may be a subsection of the Society of Telegraph Engineers, and may be able to have meetings, and make experiments, and put their forces together to endeavour to arrive at the truth. If telegraph operators wouH repeat such experiments in various parts of the world, they would give us most valuable information.
And we may hope that liesides definite information regarding atmospheric electricity, in which we are at present so very deficient, we shall also get towards that great mystery of nature —the explanation of terrestrial magnetism and its associated phenomena,—the grand secular variation of magnetism, the magnetic storms, and the aurora borealis.
NOTES We have frequently had occasion to refer to the energy and work of the Perthshire Society of Natural Science, and we rejoice to see that at its last meeting it has shown an example which we hope will be followed sooner or later by all scientific societies; it has resolved to make its influence felt in parliamentary elections. On the motion of the secretary, Dr. White, the following resolution was unanimously adopted :—" That in respect that Britain is apparently rapidly losing that commercial and manufacturing supremacy which has heretofore distinguished her, and that it is high time that the Government of this country .should take steps to retain that supremacy, and that means towards that desirable end is the appointment of a responsible Minister of Education whose duty it will be to see that our education machinery in all departments, both in extent and in efficiency, is kept up to the wants of the age, and that a thorough general education in the scientific principles on which the arts are founded (and without which training mere technical schools are of no use), is put within the reach of all, this society resolves that the candidates for the representation in Parliament of the County and City of Perth be respectfully requested to state, whether, in the event of their being elected they will use their influence to urge upon the Government; (1) the appointment of such responsible Minister of Education; (2) the promotion of scientific exploration expeditions, such as that of an Arctic expedition which the late Government was in vain requested to promote; (3) the providing of means for carrying on unremunerative scientific research." The secretary was accordingly directed to communicate with the candidates.
The post of Hydrographer to the Navy has been bestowed by Mr. Goschen on Capt. J. O. Evans, R.N., C.B., F.R.S., in succession to Rear-Admiral Richards, C.B., F.R.S., who has retired.
The first four wranglers on this year's Cambridge Mathematical Tripos, are, George C. Calliphronas, of Gonville and Caius College; Walter W. R. Ball, of Trinity College; James R. Harris, of Clare College; and Andrew Craik, of Emmanuel College.
The following lectures in Nat ural Science will be given at Trinity, St. John's, and Sidney Sussex Colleges, during Lent Term, 1874 :—On Sound and Light (for the Natural Sciences Tripos), by Mr. Trotter, Trinity College, in Lecture-room No. 11 (Monday, Wednesday, Friday, at II, commencing Wednesday, Feb. 4); On Electricity and Magnetism (for the first part of the Natural Sciences Tripos and the special examination for the Ordinary Degree), by Mr. Trotter, Trinity College, in Lecture Room No. 11 (Tuesday, Thursday, Saturday, at II, commencing Thursday, Feb. 5); On Inorganic Chemistry, by Mr. Main, St. John's College (Tuesday, Thursday, Saturday, at 13, in St. John's College Laboratory, commencing Thursday, January 29). Attendance on these lectures is recognised by the University for the certificate required by medical students pre. vious to admission for the first examination for the degree of M.B. Instruction in practical chemistry will also be given. On Paleontology (the Annuloida, &c), by Mr. Bonney, St. John's College (Tuesdays and Thursdays, at 9, commencing Tuesday, February 3). On Geology (for the Natural Sciences Tripos, Physical Geology), by Mr. Bonney, St. John's College (Mondays, Wedne?days, and Fridays, at 10, commencing Wednesday, February 4). Lithology: demonstrations with the microscope every Saturday at II, commencing February 7. The class will be limited to six, and preference gived to members of the above colleges. Elementary Geology (for the First Part of the Tripos and the special examination) (Tuesdays and Thursdays, at 11. commencing Thursday, February 5). On Botany, for the Natural Sciences Tripos, by Mr. Hicks, Sidney College (Tuesday, Thursday, Saturday, at n, in Lecture-room No. 1, beginning on Tuesday, February 3). The lectures during this term will be on Vegetable Histology and Physiology. A Course of Practical Physiol gy, by the Trinity Prelector in Physiology (Dr. Michael Foster) at the new museums. Lectures on Tuesday, Thursday, Saturday, at 12, commencing Tuesday, January, 27. This course is a continuation of that given last term.
Dr. Schmidt, Professor of Astronomy' in the University of Athens, has just complete! his great map of the Moon. It is
two metres in diameter, and is a marvel of accurate mapping and minute draughtmanship. The shading is so exquisite that any pin of the map may be examined by a lens without the appearaut of coarse or rough work. The map represents the labour of thirty-four years, and is without doubt one of the greatest asircnomical results of the century.
The discourse at the Royal Institution on Friday next, Fra. 6, at 9 P.m., will be by Mr. A. H. G.irroJ, Fellow of St. Jok'i College, Cambridge, " On the Heirt and the Sphygraograph.'
A Marine and Fresh-water Aquarium is to be established in the Central Park, New York, in connection with the Free Museum and Menagerie already erected there; it will be placed under the superintendence of Mr. \V. Saville Kent, F.Z.S., who was, until a short time ago, Curator of the Brighton Aquarium. It is intended to raise the requisite funds by public subscription, and we are very pleased to be able to add that it is proposed to endow the Institution, so that it may be made available for the purposes of scientific research.
A Project is on foot for the erection of a public aijuarium at Liverpool, and a Company has liecn formed for this purpose; a suitable site has been secured close to the Philharmonic Hall, and operations will, we believe, be commenced at once. It is estimated the building will cost about 45,00a'.
The exhibition of appliances for the economic consumption of coal, which has been formed in the Peel Park, Salford, by the Society for Promoting Scientific Industry, was formally opened on Friday. Mr. J. Lowthian Bell, who had been announced 10 open the Exhibition, was prevented from being present, but forwarded the copy of an address which he had intended to deliver. This was read by the secretary, Mr. Larking The Exhibition will remain open for some "weeks, and wil doubtless receive its share of public notice when the election are occupying less attention than tlic-y are at present.
We learn from the Athcnxum that the Trustees of the Brili''; Museum have agreed to resign their patronage into the hands of the Government.
An interesting peculiarity in the habits of some Indian Siluroid fishes has been noticed at a recent meeting of the Zoological Society by Surgeon F. Day, which will be described in full in the forthcominj Pitt of the Proceedings of tlul Society. Mr. Day, when fishing at Cassegode, found that, after having caught a large number of specimens of various species of Arias and Vsteogeniosus, there were sevsrj! silnr.>iJ eggs at the bottom of the boats, and in the fish-baskes. These eg^s were, on an average, half-an-inch in diameter; and on looking into the mouths of sever.U of the males of both genera, from fifteen to twenty eggs were seen in each; those in the boats and baskets having evidently dropped out from a similar sittiati-'n. The eggs were in different stages of de\ elopment, some advanced so far as to be just hatched. They filled the mouth, extending as far back as the branch i;e. No food was found in the alimentary canal, though in the females it was full of nutriment.
In a paper on the Meteors of January 2, rjad before the American Philosophical Society, by Prof. Daniel Kirkwood, the author states, founding on data extending from A.D. S49 to 1S64, that the meteors of this group have probably a period of thirteen years; that the mean distance is 5 53, aphelion 10.06; and that the source of the meteors may be the fourth comet of iSfio, which in its ascending node approaches very near the point passed by the earth about January 3. If the period be thirteen years, the comet should have returned in the latter part of 1S73, and the maximum fall of the associated meteors should occur about 1877.