I hope, be shortly published, go a long way towards proving it. Accepting this hypothesis, the next question we have to decide is whether the rising of the land jis an absolute or a relative rising; whether, in fact, the earth's periphery as a whole is undergoing enlargement or contraction, is stretching or shrinking. To decide this by direct observation is not easy; for water being our only measure, the same effect will be produced either by the sinking of the one portion or the rising of the other, that is, of course, if the rising or sinking be general; while if it be a local rising at one place, it may (as is familiarly known, and as I shall point out presently) be due to the lateral pressure caused by an adjacent subsiding area. In the absence of direct experiment, we may be guided by analogies from other facts. These facts are of two kinds-astronomical and geological. Since the days of Laplace, the nebular hypothesis has been generally received by astronomers, as the one which best meets observed facts. This hypothesis predicates the existence of gravitation everywhere, and shows how, by its influence, the various heavenly bodies have become condensed from nebular matter. It predicates that this force is still active everywhere, and that everywhere within our observation we have a condensation of matter in progress, matter condensing from a highly diffused condition to one of greater density. Thus each member of our own system, it is argued, is gradually and surely nearing the sun, and at the same time is shrinking, and the various planets are, in fact, in so many stages of evolution, and exhibit for us the various phases which the earth has passed through and will pass through before it is landed in the sun. This is all very elementary. I quote it only to show that the evidence of astronomy is that the earth is contracting, that its periphery is diminishing in area, and that therefore it is probable that the subsidence of the ocean-bed is absolute, while the upheaval of the land is relative only. Geologists argue differently and yet come to the same conclusion. They argue that the original condition of the earth was an incandescent one, and that it has assumed its present form after a gradual cooling, that is a gradual contraction. In Mr. Geikie's words, recently reported in your pages, "Among the geologists of the present day there is a growing conviction that upheaval and subsidence are-concomitant phenomena, and that viewed broadly, they both arise from the effects of the secular cooling and consequent contraction of the mass of the earth." The evidence of geology, then, is at one with that of astronomy in making the shrinking of the earth absolute and not relative merely. Now it is very clear that if the shrinking earth acquired a certain amount of rigidity, such shrinking would cease to take place uniformly, and the crust would give way along certain weak lines, and that corrugations, i.e. mountain-chains, and deep pits, or ocean hollows, would be formed; and not only so, but the sinking of a given area would give rise naturally to a certain thrust upwards of a contiguous area. To quote the graphic words of Mr. Geikie: "Some portions have sunk more than others. These having to accommodate themselves into smaller dimensions would undergo vast compression and exert an enormous pressure on the more stable tracts which bounded them. It could not but happen that after long intervals of strain, some portions of the squeezed crust would at length find relief from this pressure by rising to a greater or less height according to their extent and the amount of force from which they sought to escape.' From this we may conclude (what I have not seen mentioned elsewhere), that from the contraction of the earth alone we may deduce the result that the land areas have been gradually growing larger and the ocean areas smaller; that originally when the crust was less rigid, its surface was almost uniformly level and covered with water, and that as it gradually became corrugated, the land first appeared as an archipelago of islands which were gradually joined together into continents in the way Australia was clearly constructed, comparatively recently; or in other words, that the proportion of subaerial to sub-aqueous deposits must diminish as we recede in geologic time, inasmuch as the area of sea, i.e. of water-covered surface, increases. In this statement of the gradual shrinking of the earth there is little that is new, and if it accounted for all the facts I should not have troubled you with another letter. It has been taken for granted hitherto, if I be not mistaken, that areas of subsidence and upheaval are scattered about the world in a sporadic manner, with as little order and aim as plums in a pudding; that the earth being in process of shrinking, areas of subsidence occur at any point where the earth's crust is weak; but the evi dence which I have collected and which I hope the Geographical Society will publish, goes far to show that these areas are not sporadic but continuous, and further, that the foci of upheaval are in the circumpolar regions. That it is there where we meet with proofs of current and rapid upheaval almost at every step, and the farther we go north or south from the equator the more rapid does the rise seem to be, while in the equatorial regions the land masses are to a great extent quiescent; we cannot resist the conclusion that the earth is stretching itself in the direction of its shortest axis, that its periphery is being thrust out in the direction of the Poles. Now as we have shown that the earth is absolutely shrinking and that when any local uprising occurs it is due to the lateral pressure caused by a subsiding area, it becomes interesting to inquire what kind of strain upon the earth would produce a squeezing of it out in the direction of the Poles. I can see only one explanation, namely, that the strain is being applied in the way of a stricture about the world's equatorial region, that it is girdled in that region by some force which is tightening upon it, and this tightening produces a partially compensating protrusion of the two polar regions. I conceive that in a spheroid constructed of partially elastic materials, the effect of such a stric. ture will cause, besides a sensible diminution of the whole periphery of the sphere, a lateral thrust at right angles to the pressure applied, and thus only can I account for it. This would, if I am not mistaken, have another effect, and this a very important one; it would induce magnetism in the earth, and that magnetism would have its poles in the regions of upheaval, and this is in fact so. The magnetic poles are strictly, so far as our evidence goes, in the very foci of upheaval of the circumpolar regions. This correlation of terrestrial magnetism with the force that is causing a tension about the earth's equator, if sustained would surely go far to explain that crux of physical science referred to by Sir William Thomson in his address to the British Association at Edinburgh, namely, the cause of the earth's magnetism; but my letter has already outgrown reasonable limits, and I must ask you to allow me to continue the subject in another. HENRY H. HOWORTH Derby House, Eccles, Jan. 2 Vivisection It has been suggested that the study of Huxley's "Elementary Physiology" is likely to make children indulge in cruelty. Allow me to give the experience of the father of five boys on the subject. Those old enough to be taught from that book are so; and have attended the professor's lectures and seen some of his experiments. The impression left on their minds, from the reverent and touching treatment of the subject by the able professor, has led to an improved and exalted respect for the rights and life of the meanest thing that crawls. Although these boys are now at what may be called the "predatory age," the interest and respect they evince for animal life is mainly to be attributed to the beautiful and refining lectures of the worthy and humane Huxley. London, E.C., Jan. 5 Moraines G. W. COOKE MR. FRY, writing in NATURE (vol. ix. p. 103), says that "a glacier which has retreated from its terminal moraine is always the source of a stream of water, and this stream always cuts through the terminal moraine. He infers from this that a lake cannot be formed by a moraine damming up a valley. I can assure him that this is a fact which at least admits of exceptions. The valley of the Kander in the Bernese Alps is, in its upper part at least, full of the moraines of extinct glaciers, now mostly overgrown with pine forest. One of these dams up a side valley and forms the beautiful Oeschinen Lake. The lake is fed from the glaciers of the Blumlis Alp, and its water is consequently muddy. Except in most unusual floods, it has no outlet above ground, but the side of the dam farthest from the lake is one mass of springs of water as clear as the celebrated streams of Lauterbrunnen, which are evidently fed by the water of the lake filtering through the dam. The dam, being a moraine, is of porous material. JOSEPH JOHN MURPHY Old Forge, Dunmurry, Dec. 24, 1873 Indian Snakes I HAVE just had the opportunity of examining the cobra mentioned in my letter dated 12th inst., together with a very handsome one belonging to another snake charmer. This latter cobra also devoured a frog in the space of a minute or two after it was placed in the basket, the frog croaking audibly about half a minute after it was swallowed. I append the description of these cobras for the benefit of those interested in such matters. Naja tripudians.—Specimen A.—Colour above very pale olive with pair of conspicuous white, black-edged spectacles. A pair of black H-shaped marks on 12th, 13th, and 14th series (transverse) corresponding to spectacles. Posterior edges of hood above, dark olive. Blackish band 17th to 21st ventral and corresponding scales-rest of belly mottled with dark spots. Lower anterior temporal in contact with three (3) other temporals. Ventrals 182, sub-caudals 51, scales 23 series. Belly Specimen B.-Colour above, olive brown, with numerous pale olive irregular transverse bands and blotches. mottled and barred with blackish. A pair of snow-white, black-edged spectacles. Interstitial skin of anterior central portion of hood pure white, scales pale olive; that of posterior portion and margins black, scales dark olive; colour of hood extending across back in strong contrast to the paler hue of the body. A pair of white dark-edged spectacles beneath the hood, corresponding to pair above, but the white portion very much wider. Central spots below oval, black, situated on 10th, 11th, and 12th series of scales. Scales of head pale olive, anterior margins of vertical, supraciliary and occipital shields dark olive, forming a double band across the head. Posterior margins of occipitals dark olive. A vertical infra-orbital streak of dark olive. Lower anterior temporal in contact with three (3) other temporals. The following ventrals blackish, forming distinct bands 17th to 31st, 24th to 30th, 35th to 38th, 48th to 51st, 61st to 64th all inclusive. Beyond these there are dark bands but the ventrals composing them are not as a rule black throughout. Ventrals 185, sub-caudals 53, scales 23 Sept. 17, 1873 E. H. PRINGLE The use of Terms in Cryptogamic Botany Ir seems to me that there is a very perplexing want of uniformity in the names employed by different authors to indicate the reproductive organs of cryptogamic plants. To a private student this want of formality in the nomenclature of homologous organs is very bewildering; especially when he happens to meet with a term which no botanical work or glossary within his reach explains. In reading the Rev. M. J. Berkeley's "Introduction to Cryptogamic Botany," I have come across a term which I cannot find used in the same sense in any botanical work I have consulted. In the division of algae called Rhodospermeæ, he says, in speaking of the fruit, "indefinite spores in distinct nuclei." In Callithamnion corymbosum he calls the expanded wall of the mother cell from whose endochrome the walls have been produced by cell division, the nucleus. In some other genera, he calls the cluster of naked sporethreads the nucleus. In other genera the spore threads arising from a placenta, together with the conceptacles containing them are called a nucleus. In Wrangelliacea it is stated that the nucleus is composed of pyriform spores arising from the endochromes of the terminal cells of the spore-threads. I had first settled in my mind that nucleus was used by Mr. Berkeley as a general name in this disvision of algae, for an indefinite cluster of spores. On re-consideration it seemed to me that the term nucleus in the division Gongylospermea was not applied to the clusters of spores, but to the expanded wall of the mother-cell, or walls of the mother-cells, whose contents had been transformed into spores; and in the great division Desmiospermea to the spore-threads from whose cells the spores are produced. Having at length given up this supposition as untenable, it then occurred to me that "nucleus" did not mean the expanded walls of the mother-cells alone, or the clusters of spores alone, or the spore-threads alone; but was a general term applied to the fruit consisting in some cases of spores and spore-threads, in others spores, spore-threads and conceptacles, and in others of the expanded walls of the mothercells and their contained spores. When, however, I again read that in Wrangelliacea the nucleus is composed of radiating pyriform spores, I gave up all attempts at a solution satisfactory to myself. Can any of your readers inform me what, in this division of algae, is meant by the term "nucleus," and why it is only used in this division? Did the term not occur in a book written by so high an authority in Cryptogamic Botany it might be passed over D. R as a piece of affectation on the part of the writer. POLARISATION OF LIGHT* III. WE E now proceed to the consideration of the colours produced by plates of crystal when submitted to the action of polarised light. A crystal very commonly used for this purpose is selenite or sulphate of lime, which is readily split and ground into flat plates of almost any required thickness. If such a plate be placed between the polariser and analyser when crossed, it will be found that there are two positions at right angles to each other, in which, if the selenite be placed, the field will remain dark as before. The selenite is, in fact, a doubly refracting crystal, and the positions in question are those in which the plane of vibration of the ordinary ray coincides with that of the polariser (or analyser), and that of the extraordinary ray with that of the analyser (or polariser). In every other position of the selenite, and notably when it has turned through 45° from either of the positions before mentioned, or neutral positions as they may be called, light passes through, and the field becomes bright. If the thickness of the selenite be considerable, the field when bright will be colourless; but if it be inconsiderable, say not more than three millimetres, the field will be brilliantly coloured with tints depending upon the thickness of the plate. Supposing however that, the selenite remaining fixed, the analyser be turned round, we shall find that in the first place the colour gradually fades as before; until when the analyser has been turned through 45°, all trace of colour is lost. After this, colour again begins to appear; not however the original tint, but its complementary; and in fact, there is no more sure way of producing colours complementary to one another than that here used. A general explanation of this change of colour is already furnished by our former experiments. Doubly refracting crystals generally, in the same way as Iceland spar, divide every ray, and consequently every beam of light which passes through them, into two, so that of every object seen through them, or projected through it on to a screen, two images are produced. These two, being parts of one and the same beam of light, would, if recombined, reproduce the original beam; and the same is, of course, the case with the two images. This may be rendered visible by using the double-image prism as an analyser, and throwing both images on the screen together. As the prism is turned round, it will be seen that, just as when no selenite was interposed, the images are alternately distinguished; but that when both are visible, their colours are complementary. And if the distance of the prism be so adjusted that the images overlap, it will be found that, when both are visible, the part where they overlap is always white, whatever be the thickness of the plate used. An instructive experiment may be made by interposing an opaque object in the path of the beam of light, so that its shadow may fall upon the part of the field common to the two images. The shadow will of course intercept the light forming each of the images, and will consequently Suppose that the two images are appear double. *Continued from p. 169. coloured red and green respectively; then one of the shadows will be due to the shutting off of the red light, and the other to that of the green. But in the first case the space occupied by the shadow will be still illuminated by the green light, and in the second by the red. In other words, neither of the two shadows will be black, one will be green, and the other red. If in any part of their extent the two shadows overlap, the part common to the two, being deprived of both red and green light, will be black. But in order to explain how it comes to pass that colour is produced at all, as well as to find a more strict proof that the colours of the two images are complementary, we must have recourse to some considerations based upon the wave theory of light. And first as to the mode in which waves may be produced. Consider a row of balls lying originally in a horizontal straight line. Let the balls start one after another and vibrate at a uniform rate up and down. At each moment some will be at a higher, others at a lower level, at regular intervals in a wave-like arrangement; the higher forming the crests, the lower the hollows of the waves. The distance from crest to crest, or from hollow to hollow, is called the wave length. The distance from crest to hollow will consequently be half a wave-length. This length will be uniform so long as the vibrations are executed at a uniform rate. Each ball in turn will reach its highest point and form a crest; so that the crests will appear to advance from each ball to the next. In other words, the waves will advance horizontally, while the balls vibrate vertically. If the row of balls were originally arranged in a wave form, and caused to vibrate in the same way as before, those on the crests would vibrate wholly above, and those in the hollows wholly below the middle line. When the balls originally on the crests rise to their highest points, those in the hollows will fall to their lowest positions, and the height of the wave will consequently be doubled. When the balls originally at the crests fall, those in the hollows will rise, both to the middle line; and the wave will consequently be annihilated. The first of these corresponds to a condition of things wherein the crests of the new wave motion coincide with those of the old, and the hollows with the hollows; the second to that wherein the crests of the new coincide with the hollows of the old, and vice versa. Hence, when two sets of waves are coincident, the height of the wave or extent of vibration is doubled; when one set is in advance of the other by half a wave length, the motion is annihilated. The latter phenomenon is called interference. When one set of waves is in advance of the other by any other fraction of a wave-length, the height of the wave, or extent of vibration, is diminished, but not wholly destroyed; in other words, partial interference takes place. The distance whereby one set of waves is in advance of another is called the difference of phase. The Wave Theory of Light consists in explaining optical phenomena by vibrations and waves of the kind above described. And according to that theory the direction in which the waves move is the direction of propagation of the ray of light. The intensity of light depends upon the extent of the vibrations or the height of the waves; the colour upon the number of vibrations executed in a given interval of time. And since throughout any uniform medium the connection of the parts and the rate of propagation may be considered to be uniform, it follows that the waves due to the slower vibrations must be longer than those due to the more rapid. Hence the colour of the light may be regarded as depending upon the wave length. The substance to the vibrations of which light is supposed to be due, is an elastic fluid or medium pervading all space, and even permeating the interior of all bodies. A full statement of the reasons which have led philosophers to make this hypothesis would involve considerations derived from other sciences besides optics, and would be out of place here. But it may still be pointed out that one strong argument is furnished by the fact of the transmission of light from the sun and from the fixed stars through space, where no atmosphere or known gases can be conceived to exist. That the light so traversing interstellar space must be transmitted by a material substance, is a fundamental proposition of mechanical philosophy; and the hypothesis of the ether simply consists in attributing to the substance or medium the property of elasticity (a property possessed in a greater or less degree by all known bodies), without assuming anything else whatever as to its nature or relation to other substances. In the illustrations of wave motions given above, the balls would represent successive portions or molecules of the ether; and the means whereby the motion of one molecule is transmitted to its neighbour, is the elastic cohesion attributed to the whole medium in the hypothesis above mentioned. The difference between ordinary and polarised light has been explained above; and the mechanical contrivances devised for representing wave motion always have reference only to polarised light. But as this is the subject with which we are here concerned, the limitation in question is not of consequence. A variety of instruments have been constructed for showing the effects of compounding vibrations or waves under different circumstances. best with which I am acquainted is that by Sir Charles Wheatstone. ray. The In plane polarised light, such as is produced by tourmalin plates, by double refraction in Iceland spar, &c., the vibrations are rectilinear, and are executed in one and the same plane throughout the entire length of the In circularly polarised light the vibrations are all circular, and the motion is performed in the same direction. In elliptically polarised light the vibrations are all elliptical, the ellipses are all similarly placed, and the motion is in the same direction for the entire ray. These are the only known forms of polarisation, and indeed they are the only forms compatible with the usual, simplest assumption respecting the elasticity of the ether. These general considerations being premised, we are in a position to trace the course and condition of a ray of light issuing from the lamp or other source, and traversing first the polarising Nicol's prism; secondly, the plate of doubly refracting crystal; thirdly, the analysing Nicol. The vibrations of the ray on leaving the polariser are all restricted to a single plane. On entering the plate of doubly refracting crystal, every ray is divided into two, whose vibrations take place in planes perpendicular to one another. The angular position of these planes about the axis of the beam of light is dependent upon the angular position of the crystal plate about its centre. The two sets of rays traverse the crystal with different velocities, and therefore emerge with a difference of phase. The amount of this difference is proportional to the thickness of the plate. On entering the analyser the vibrations of each pair of rays are resolved into one plane; and are then in a condition to exhibit the phenomena of interference. If the plane of vibration of the analyser be parallel to one of those of the plate, that ray will be transmitted without change; the other will be suppressed. In any other position of the analyser those monochromatic rays (spectral components of white light whose difference of phase most nearly approaches to half a wave-length, will be most nearly suppressed; and those in which it approaches most nearly to a whole wavelength will be most completely transmitted. The amount of light suppressed increases very rapidly in the neighbourhood of the ray whose difference of phase is exactly a half wave-length; so that with plates of moderate thickness a single colour only may in general terms be considered to be suppressed. This being so, the beam emergent from the analyser will be deprived of that colour, and will in fact consist of an assemblage of all others; or in other words will be of a tint complementary to that which has been extinguished. Next, as regards the colours of the two images, that is, the two which are formed either simultaneously by a double-image prism or successively by a Nicol in two positions at right angles to one another. In the first place it is to be remembered that the two sets of vibrations into which the selenite has divided the polarised ray are at right angles to one another; secondly, that one set is retarded behind the other through a certain absolute distance, which is the same for every ray, and consequently through a distance which is a different fraction of the wave-length for each colour; thirdly, that these two are re-combined or "resolved " in a single direction in each image by the analyser. This being so, bend two wires in the following form : : N" and place them at right angles to one another about their middle line M N M' N'.. so that the points M of the two wires coincide, and likewise N, and so on. This will represent the condition of the vibrations as they emerge from the selenite, when the plate is of such a thickness as to cause a retardation equal to one or to any whole number of wave-lengths. Turn the wires about their middle line M N M'N' until they meet half way, i.e. in a position inclined at 45° to their original directions; this will represent the vibrations as resolved by the analyser in one image. Turn the wires about their middle line as before, but in reversed directions, until they meet in a position at right angles to the former; this will represent the vibrations as resolved by the analyser in the other image. On looking at the wires when so brought together, it will be found that in one case the crests fall upon the crests and the hollows upon the hollows, so that the vibrations combine to increase the intensity of the light. In the other case the crests fall upon the hollows and the hollows upon the crests, so that the vibrations interfere and completely neutralise one another. The same principle would obtain if we shifted one wire along the middle line so that the points M of the two wires no longer exactly coincide. This would represent the condition of the vibrations as they emerge from the selenite when the plate is of such a thickness as to cause a retardation of a fraction of a wave-length equal to the amount of shift. And on turning the wires as before, we should find that in one image the waves partially combine, and that in the other they partially interfere. The shifting of the wires would represent either the effect of plates of different thickness upon waves of the same length, i.e. rays of the same colour; or that of a single plate on waves of different lengths, i.e. on rays of different colours. From these considerations we may conclude that the rays which are brightest in one image are least bright in the other; or, in other words, that the colours of the two images are complementary. It has been remarked that the colour produced by a plate of selenite depends upon the thickness of the plate. În fact, the retardation increases with the thickness, and consequently, if, for a given thickness, it amounts to a half wave-length of the shortest (say violet) waves, for a greater thickness it will amount to a half of a longer (say green) wave, and so on. And if, instead of a series of plates of different thicknesses, we use a wedge-shaped plate, the entire series of phenomena due to gradually increasing retardation will be produced. This is easily seen to consist of a series of tints due to the successive extinction of each of the rays, commencing with the violet and ending with the red. And the tints will consequently have for prevailing hues the colours of the spectrum in the reverse order. This series of colours will be followed by an almost colourless interval, for which the retardation is intermediate between a half red-wave length and three half violet-wave lengths. Beyond this again the series of colours will recur; and the same succession is repeated as the wedge increases in thickness. It will, however, be observed that the colours appear fainter each time that they recur, so that when the thickness reaches a certain amount (dependent upon the nature and retarding power of the crystal) all trace of colour is lost. It is not difficult to account for this gradual diminution in the intensity of the colours if, by means of a diagram, we examine the mode in which the waves of various lengths interfere with one another; but spectrum analysis furnishes an explanation which is perhaps more easy of general apprehension. If the light emerging from the analyser be examined by a spectroscope, it will be found, in the case of a plate giving the most vivid colour, that the spectrum presents a dark band indicating the colour which has been extinguished. On using thicker and thicker plates the band will be found to occupy positions nearer and nearer to the red end of the spectrum, until the band finally disappears in the darkness beyond the least refrangible rays that are visible to the eye. If the analyser be turned round the band will gradually lose its characteristic darkness, until, when the angle of rotation has reached 45°, the band will have disappeared altogether. The spectrum is then continuous, and when recompounded will give white light. This corresponds to the fact noticed before, that when the analyser is turned round, the colour given by a selenite plate fades and finally disappears when the angle of rotation amounts to 45 If the rotation be continued a band reappears, not, however, in its original position, but in the part of the spectrum complementary to the former. bands will be seen instead of one; with a still greater If the thickness of the plate be further increased, two thickness there will be three bands, and so on indefinitely. The total light then of which the spectrum is deprived by the thicker plates is taken from a greater number of its parts; or in other words, the light which still remains is distributed more and more evenly over the spectrum, and consequently at each recurrence of the tints the sum total of it approaches more and more nearly to white light. The following experiment will be found very instructive. Take two wedges of selenite or other crystal, and having crossed the polariser and analyser, place the two wedges side by side in the field of view so as to compare the tints produced by the two. Then place one over the other, first with the thick end of the one over that of the other; next with the thick end of the one over the thin end of the other. If the two plates are exactly similar, the combination in the first instance will be equivalent to a single wedge whose refracting angle is double that of a single wedge; and the number of bands produced will consequently be doubled. In the second combination the angles of the wedges will compensate one another, and the result will be equivalent to a uniform plate whose thickness is equal to the sum of the mean thicknesses of the wedges. The field will then be coloured with a uniform tint, viz., that due to a plate of the thickness in question. By making use of the principle that the colour produced depends upon the thickness of the plate, selenites have been cut of suitable shapes and thicknesses, so as to produce coloured images of stars, flowers, butterflies, and other objects. W. SPOTTISWOODE (To be continued.) ON THE MOTION AND SENSATION OF IT is needless for me to say to the ladies and gentlemen of Sound. Now every boy knows what I mean when I speak of the sensation of sound. The impression, for example, of my voice at the present time upon the organ of hearing is the sensation of sound. But what right have I to speak of the motion of sound? This point must be made perfectly clear at the beginning. For this purpose I will choose from among you a representative boy, or allow you to choose him, if you prefer doing so. This boy, whom you may call Isaac Newton, or Michael Faraday, will go with me to Dover Castle, make the acquaintance of the general commanding there, Sir Alfred Horsford, and explain to him that we wish to solve an important scientific problem. He is sure to help us: he will lend us a gun, and an intelligent artilleryman; and we will make arrangements with this man to fire the gun at certain times during the day. We set our watches together; and now, before quitting him, we ask the artilleryman to fire one shot. We are close at hand, and we observe the flash and listen to the sound., There is no sensible interval between them. When we stand close to the gun flash and sound occur together. Well, we quit the artilleryman, warning him to fire at the exact times agreed upon. Let us say that the first shot is to be fired at 12 o'clock, the second at 12.30, and so on every half hour. We quit our artilleryman at half past eleven, descend from the castle to the sea-shore, where a small steamer is awaiting us. We steam out a little better than a mile from the place where we have left the artilleryman; and now we pull out our watches and wait for 12 o'clock. Newton at length says, "In exactly half-a-minute the gun ought to fire ;" and, sure enough, at the exact time agreed upon, we see the flash of the gun. But where is the sound which occurred with the flash when we were on shore? We wait a little, and precisely five seconds after we have seen the flash we hear the explosion; the sound having required this time to travel over a little better than a mile. We now steam out to twice this distance and wait for the 12.30 gun. We see the flash, but it requires ten seconds now for the sound to reach us; we treble the distance, it requires fifteen seconds; we quadruple the distance, and find the sound requires twenty seconds to reach us. And thus, if the day were clear, we might go quite across to the coast of France and hear the gun there. In all cases we should find that the flash appeared at the precise time agreed upon with the artilleryman, which proves that light reaches us in so short a time that our watches fail to give us any evidence that the light requires any time at all to pass through space, while the sound reaches us later * Royal Institution Christmas Lectures, 1873-4, by Professor Tyndall, D.C.L., LL D., F.R.S. These lectures have not been written out, much less intended for publication. At the request of our Reporter, Dr. Tyndall has consented to their appearance in NATURE. and later the farther we go away. I think these experiments give us every right to speak of the "Motion of Sound.' But they also inform us how the velocity of sound has been actually determined. The most celebrated experiments on this subject have been made in France and Holland. Two stations were chosen ten or twelve miles apart; guns were fired at each station, and the interval between the flash and the report was accurately measured by the observers at the other station. In this way it was found that when the air is at the temperature of freezing water, the velocity of sound through it is 1,090 feet a second. On different days we should find it travelling at different speeds-as the weather grows warmer the sound is found to travel faster. But I must not let you go with the idea that light re |