Thus the plus sign preponderates, indicating greater warmth above, during the day and night, in January, February, October, November, and December, and during the night throughout the year.

A second thermometer, properly protected from radiation, was placed in the middle of the year 1869 at the height of 50 feet; and since then its readings have been regularly taken. The mean monthly temperatures of the air, at 50 feet height, were found to differ from those at 4 feet, as follows:

Thus we have the unexpected result that the mean monthly temperature of the air at 22 feet and at 50 feet height is higher during the evening and night-hours throughout the year than at the height of 4 feet, and also higher, night and day, during the winter months. By selecting those days with a sky covered by dense clouds, it was found that there was on such days no difference between the temperature at 4 feet, 22 feet, and 50 feet height. At the height of 50 feet, in the summer months, the temperature during the day was frequently 6 and 7 degrees lower than that at 4 feet, and at night 5 or 6 degrees higher.

On a New Electro-Magnetic Anemometer, and the mode of using it in Registering the Velocity and Pressure of the Wind. By JOHN J. HALL. After describing at some length the difficulties attending the use of the present forms of anemometrical apparatus, arising from the fact that few houses are built with any means of access to the roof, also from the interference of trees and undulatory surfaces of land, &c., and having showed the practical results that would be derived from the use of electricity, the author proceeded to describe the apparatus devised (and exhibited) by him. One of the main objects for which it is intended is the determination of interval or hourly velocities. The following is a brief résumé of its principles and construction:

The anemometer consists of two parts, viz. Velocity apparatus and Registering apparatus.

The first consists of a set of Robinson's hemispherical cups, which communicate their motion downwards into a brass box, where it is reduced in angular velocity and causes a contact-disk or commutator (in which two platinum contact-pins are fixed equidistant from one another) to revolve in tenths of a mile. An insulated metallic lever, having a platinum working face, stands on each side of the disk so that, upon the completion of every mile, one or other of the contact-pins comes into contact with the two levers, thus uniting them and completing the circuit. The levers, which are jointed at their opposite ends, are raised a few degrees (of circles, whose radii they represent), and then fall back to their normal position, ready to be taken up by the next pin, and so on.

The Recording apparatus consists of a train of wheels and pinions working in a frame or between two brass plates, the arbors of which project through a dial-plate (whereon the circles and figures are engraved), and carry the hands. These wheels are driven by a weight attached to a line which is wound round a barrel; and a locking pin-disk (the pinion of which works in the first wheel) is released at every contact of the cup apparatus by an electro-magnet, which unlocks the pin-disk and allows the first hand to advance mile on the graduated dial by a jump similar to the minute-hand in remontoire clocks.

By turning "on" a "strike-silent" stop, a hammer lever is brought into connexion with the escapement, and strikes a ball at every contact; the observer has therefore nothing to do in noting interval or hourly velocities but to notice the seconds' hand of his watch or chronometer (a split-seconds or chronograph would be preferred), while he counts the number of times the bell is struck, each of which corresponds to the mile, and, by formulæ arranged and explained by Mr. Hall (who has also arranged a comprehensive series of tables for use with the instrument) the hourly velocity may be readily deduced.

The following formula has been arranged for deducing the hourly velocity of the wind from observations during intervals of minutes and seconds.

Let T be the interval of observation in minutes and seconds, expressed decimally, 60 constant (min.-1 hour), and r the quantity required, which will represent the number of times T is contained in one hour, 05 unit of distance, b number of beats on bell, r as before, and V velocity required; then

Supposing, therefore, the bell is struck 15 times in 1 min. 30 sec., expressed decimally 1.50 min.; then

By noticing the exact seconds upon which the first and last beat are struck, the results will be as accurate as if the instrument were capable of recording the onethousandth part of a mile, while the battery is less called into action.

In noting velocities extending over long periods of time, the instrument is read in the same manner as the ordinary cup-and-dial anemometer.

This paper, which was of considerable length, was illustrated by the Electrical Anemometer (by Messrs Negretti and Zambra, under Mr. Hall's directions) and mechanical diagrams.

On the Rainfall of the United States. By Professor J. HENRY.

HEAT, LIGHT.

Queries respecting Ether. By CHARLES BROOKE, M.A., F.R.S. When light and caloric were supposed to consist of material molecules, the hypothesis of the universal existence of a transmitting medium was unnecessary, since particles of matter might with the utmost freedom be projected through vacuous space; bnt as light and heat are now generally admitted to consist not of transmitted matter, but of transmitted vibratory motion (and why may not electricity, so freely interchangeable with the former, be admitted into the same category?), the necessity of the existence of a highly elastic and attenuated transmitting medium, pervading infinite space, becomes at once apparent; and this medium, hitherto not cognizable to our senses, has been termed "æther." But it has been further assumed that æther is alone capable of transmitting the extremely rapid vibrations of light and heat, and that it must therefore necessarily pervade or permeate all kinds of sensible matter. The questions proposed to be raised in this communication are the necessity of this interstitial hypothesis, and the probable capability of ordinary matter to transmit the vibrations of light and heat.

It is now generally admitted that when a body becomes heated, its own particles, and not merely those of the supposed interstitial æther, are thrown into a state of vibratory motion, the amount of heat corresponding probably to the amplitude of the vibrations; hence a certain amount of energy has been communicated to those particles, and (at all events, in the case of celestial radiations) the molecules of æther must previously have possessed the energy or vis viva which they have com

municated. Hence ether, being susceptible of vis viva, has recently been admitted to be ponderable; but this admission is not a necessary consequence; for although the idea of existing energy is associated with that of weight, in consequence of the constant energy acquired by gravitation having been taken as the measure or unit of energy however acquired, there is no necessary connexion between them. Suppose, for example, that a flea were placed on an orbitating planet of the size of a pumpkin; while its muscular energy would remain undiminished, its weight would be infinitesimal, and the first leap would obviously plunge it into infinite space, to perform subsequently perhaps an independent orbit.

The only basis on which the interstitial-ather hypothesis rests is the assumed incapacity of ordinary matter, whether in the solid, liquid, or gaseous state, to transmit the vibrations of light and heat, because the only vibrations hitherto recognized, namely those of sound, are almost immeasurably slower than those of light and heat; the one being numbered by at most a few thousands, the other by hundreds of millions of millions in one second of time. But it must be borne in mind that sonorous vibrations are always longitudinal, in the production of which repulsive forces are alone concerned; whilst, on the contrary, light- and heat-vibrations are necessarily transverse, and the production of these is solely due to attractive forces. Now these respective forces obey very different laws; for whilst attractive forces obey generally, and probably universally, the law of the inverse square of the distance, molecular repulsion must obviously, at all events in gaseous matter, obey the law of the inverse cube of the distance; therefore from the rate of transmission of longitudinal vibrations nothing can be predicated respecting the rate of transmission of transverse waves. It has been asserted that molecular repulsion is a dynamical resultant effect, and therefore incapable of expression by a statical law; but it is very doubtful whether molecular attraction is not equally a dynamical sequence, and therefore not a whit more entitled to claim a statical law than the former.

It has been shown from the investigations of Mr. Norman Lockyer that incandescent gases existing in the vicinity of the sun are capable of initiating vibrations of definite periods, which are moreover occasionally accelerated or retarded by the proper motion of the emitting gas, just as sound-waves have been shown by Savart to be accelerated or retarded, and the sound consequently raised or lowered in pitch, by the proper motion of the body producing the vibrations. What reason can there then be for doubting that gaseous matter is capable of transmitting heatwaves, and, if so, of likewise transmitting the waves of light, since the two are so intimately connected by the identical phenomena of reflexion, refraction, and polarization? may not, in fact, in some instances the perceptions of light and heat be but different sensuous impressions produced by the same vibrations?

Now in the denser forms of matter, namely the solid and liquid, it appears that the wave-lengths of excited transverse vibrations are indefinitely modified, probably by the more energetic action of repulsive forces; for whilst any given kind of matter in the solid or fluid state is found, when incandescent, to emit light- and heat-waves of all lengths, and so to form a continuous spectrum, the same matter in the form of incandescent gas will emit only a few sets of waves of definite and invariable lengths, forming an interrupted spectrum of bright lines; and moreover some of these wave-lengths are frequently found to bear very simple numerical ratios to each other. And even in gaseous matter it has been observed that the bright lines in the spectrum become narrower and more sharply defined by rarefaction, and, on the contrary, broader and less defined by condensation. Moreover, as regards the density of the absorbing medium, the absorption-bands in the spectrum appear to obey the same law as the bright lines. In other words, every kind of matter appears to be capable of emitting or absorbing its own peculiar waves, according to its tenuity, that is, as the results of molecular attraction are less and less interfered with by those of repulsion. The well-known peculiar incapacity of any given transcalent substance to transmit the heat-rays emitted by a heated portion of the same substance, or, in other words, the ability of the molecules to freely appropriate the wave-motion that has been induced in some intervening medium by similar molecules, seems further to argue that ordinary matter is capable of assuming vibrations having the extreme rapidity of those of light and heat; and that

there exists no valid ground for a distinction between light and heat in this respect is further confirmed by the experiments of Mr. B. Stewart, who has shown that the emission of light by incandescent bodies closely corresponds with their absorptive power (whether selective or otherwise) when not incandescent, and, further, that even in the decomposition of light into two polarized beams by the tourmaline, that substance emits when incandescent the ray that is otherwise absorbed. Can there, then, be any valid reason for doubting the ability of ordinary matter to transmit those transverse vibrations which it is obviously capable of either absorbing or emitting? and if so, what ground is there for the hypothesis that the transmission of light- and heat-waves necessitates the presence of imperceptible æther in the interstices of perceptible matter?

If the existence of æther in infinite space, essential to the undulatory theory, be admitted, it may be asked how is it possible to conceive its exclusion from any portion of space? A very simple hypothesis, propounded by the writer in the Introduction to the last edition of his Elements of Physics,' will meet this difficultynamely, that æther (like its fluid namesake with water) is immiscible with known gaseous matter. This, it must be admitted, is sheer hypothesis; but if true, it must ever remain so, æther being in that case beyond the reach of human ken: of this we may, however, rest assured, that if it be not wanted in and around even our corporeal frames, it is not there; the contrary supposition would be inconsistent with the infinite wisdom of the Creator of the universe.

On certain Objections to the Dynamic Theory of Heat.
By H. WHITESIDE COOK.

In this paper the author first endeavoured to show that heat must necessarily be a force of a permanent nature, could not possibly be a mere affection of matter, and, as is asserted by the believers in the "Dynamic" hypothesis, that it must be in the nature of an energy," and not an "impulse."

He then proceeded to analyze the nature of heat as described in the thermodynamic theory, and brought forward arguments to show that the causes which produce heat would not produce the molecular motion presumed, and that, on the other hand, allowing this molecular motion to exist, it would not produce the effects which are produced by heat. He proceeded, in conclusion, to consider one or two of the experiments on which the dynamic hypothesis was based, and showed that they were in no way incompatible with the old theory of a caloric or substantive heat. In short, the argument of the paper was:-that though such forces as electricity, magnetism, &c. were probably justly considered to be only affections of matter, it was a mistake to conclude the same thing of heat; that if the attractivity of matter be a permanent energetic force, then heat, the force which counteracts that attractivity among molecules, must also be a permanent energetic force; for a force of impulse cannot cope with a perpetual energetic force; however great the impulse, it must soon be beaten; and were heat a condition and not an entity, then it would be but a passing phenomenon.

In dealing with the experiments which are supposed to substantiate the dynamic hypothesis, the author dwelt especially on Davy's celebrated experiment of liquefying ice by friction, when he showed that the increment of heat added to the ice was very small in comparison with the amount of heat contained by the ice when at the temperature of 32 Fahr., and that the molecular agitation to which it was subjected would cause it to absorb this heat from the atmosphere of caloric, which, on the substantive theory, would ex hypothesi surround it. He next spoke of the fall of temperature which takes place when compressed air is allowed to escape from the confining vessel (an experiment which is put forward by the exponents of the dynamic theory as instancing the conversion of heat into mechanical energy), and pointed out that in the preparation of the experiment the compression of the air had forced out of it some of the heat that it contained. When, then, it was allowed to escape, the air brought out less heat than it took in, the difference being the amount which had been given out in cooling after the original compression. The author added that these experiments could not be said to substantiate the dynamic theory-and that if he did not mention more of them, it was because those present

would be able to call them to mind themselves and apply to them reasoning precisely similar.

The author attached great importance to the question, because he thought that in erroneously considering heat to be molecular vibration, we lost sight of the true explanation of electricity, which he believed to be neither more nor less than that very vibration or molecular motion which the dynamic theory called heat.

On the Wave Theory of Light, Heat, &c. By Dr. HENRY HUDSON. Huyghens (to explain double refraction) assumed a second vibrating medium as consisting of "the molecules and æther conjointly;" and Fresnel's grand theory rests on the same foundation. As molecular vibrations in air (sound-waves) are 10,000 times longer and 869,000 times slower than etherial waves, the author rejects this combination as inadequate to account for the very minute difference in the retardation of the doubly refracted rays in crystals. He then adduces several cases, especially in polarized light, unexplained on Fresnel's theory, and proceeds to show that all the difficulties in Fresnel's theory can be removed by considering the "æther" to consist of two media, each possessed of equal and enormous self-repulsion, and both existing in equal quantities throughout space, being also mutually indifferent (neither attracting nor repelling), and that their vibrations consequently always take place in perpendicular planes. He then suggests as an experimental test of this view, "that the ordinary refracted ray, through Iceland spar, cannot be made to show any phenomena of Interference with the APPARENTLY similarly polarized ray obtained by total reflexion from glass," because, on this view, their vibrations are in different media. After discussing many curious and interesting questions, he pointed out that the two Electricities fulfil the requirements of the theory, being, as he asserts, mutually indifferent, and constitute the ather. Electrical phenomena the author would explain by the existence of "waves of translation" as well as "6 waves of vibration "affecting the molecules of bodies-the former being most prominent in "statical phenomena" (induction especially), and the latter more generally observed in what is denominated "the electric current."

OPTICS.

On the Immersion Method of Illumination of the Microscope.
By Dr. JOHN Barker.

After showing the defects of the present methods as exhibiting merely shadows and caustics of reflection and refraction, and markings resulting very often from the relative opacity and transparency of the parts of an object, the author was led to believe, from the study of the way in which objects are best illuminated for unassisted vision, that this was the method to which we should endeavour to approximate in the illumination of microscopic objects. The adaptation of the immersion plan in condensers of various forms seemed to him to best fulfil these requirements. The object would be illuminated by very oblique light, the oblique rays being most economized, undergoing less loss and less dispersion. The author brought forward several forms of this mode of illumination, in which a flat-topped paraboloid was used, which, he stated, gave very good results with a two-thirds used with binocular microscopes; another was a flat-topped paraboloid to be used above the object, and in the centre of which (the glass paraboloid) the power was placed so as to light up the object under the highest powers with reflected light. The ordinary achromatic condenser, too, he thought, might be greatly increased in value by adapting it to the immersion plan.

On the New Binocular Microscope. By S. HOLMES.

The author showed that the views of objects seen through a microscope, being