the body a steady motion. Various plans have been from time to time tried, with the object of imparting to long projectiles a steady flight. They have been made with spiral grooves cut externally on their periphery, or internally from front to rear, in the expectation that the resisting action of the atmosphere acting on the inclined surfaces would give the requisite spinning motion. Again, they have been made very long, and furnished with fins or feathers, in order that they may be propelled on the principle of the arrow, but no practically successful results have as yet brought projectiles of this kind into use. The required object is, as is well known, readily and successfully effected by propelling the elongated projectile from a rifled barrel, that is, a tube having its interior made of such a spiral form that the projectile, while it is propelled from the breech to the muzzle, is turned round its axis of progression: a rotary motion is thus imparted, which is retained by the advancing projectile, and gives it the required steady motion. The elongated bullet was first used with rifled small-arms, either polygrooved or fluted, or, like the Enfield, having three grooves. The length, however, was limited; and various attempts were made to fire longer projectiles, compounded of various metals, and of various shapes, so that by changing the position of the centre of gravity they might be propelled point foremost. But if made beyond a certain length they were always found to turn over at moderately long ranges. Mr. Whitworth was the first to enunciate the principle that projectiles of any requisite length could be successfully fixed by giving them rapid velocity of rotation, which should be increased in proportion with their increased length. He, as is well known, uses rifles having a spiral polygonal bore, in which all the interior surfaces are made effective as rifling surfaces. The success of the elongated projectile having been established in the case of small-arms, their employment with ordnance followed as a natural consequence. Rifled ordnance were, therefore, called into existence to meet the requirements of the time. In fact, rifled cannon may be considered as a rifled musket made with enlarged proportions.

The importance of giving to ships intended for high speed the shape best suited to facilitate their progress through water is now universally acknowledged; and Mr. Whitworth considered that it was necessary to ascertain, by reasoning upon similar grounds, and by experimental research, what was the proper shape to give his projectile, so that it might be propelled through the air under conditions most favorable to precision and range. He, after numerous corroborating experiments, decided that the projectile adopted by him was the best. It has a taper front, having nearly the external section of what mathematicians term the solid of least resistance, the curve being somewhat rounded; the rear is made to taper in such proportion that the air displaced by the front is allowed readily to close in behind upon the inclined surfaces of the rear part. The middle part is left parallel to the required distance, to provide rifling surfaces, and obviate windage. The results of long and repeated trials show that this form of projectile gives much greater precision and a superiority of range, varying from fifteen to twenty-five and thirty per cent. (according to the elevation and consequent length of range), as compared with a projectile of the common rounded front and parallel rear end. At low elevations,

where the range is comparatively short and the velocities great, the difference in the result of the taper and non-taper rear is not so marked as at the higher elevations, where the mean velocities of the projectiles are reduced. But at all ranges the superiority exists both in precision and velocity, as the elongated projectile, at no practical range, has a mean velocity so great as to prevent the atmosphere closing in behind it. One of the most important advantages attending the use of the taper rear is, that it gives a lower trajectory, which renders errors in judging distance of minor importance, as the projectile which skims along near the ground is more likely to hit a mark, especially a moving one, than a projectile which, moving in a more curved path, has to drop, as it were, upon the object aimed at, whose distance therefore must be accurately guessed. The taper shape of the rear is peculiarly well adapted for the proper lubrication of the gun, which is most essential for good shooting.

Various forms of elongated Whitworth projectiles, suited for special purposes, were described: tubular projectiles for cutting cores out of soft materials, as the sides of timber ships; flat-fronted, hardened projectiles, first used by Whitworth, and afterwards by Armstrong, for penetrating iron plates. It is found that these projectiles penetrate, when fixed point blank, through iron plates inclined at an angle of fiftyseven and a half degrees to the perpendicular. The edge of the flat front, though slightly rounded, takes a hold, as it were, as soon as it touches the plate, and the resistance met is merely that due to the thickness of plate, measured diagonally. Official experimental trials made on board the Excellent at Portsmouth showed that these projectiles penetrate readily through water, and would go through a ship's side below water-mark.

THE MOTION OF CAMPHOR UPON WATER.

Mr. Charles Tomlinson has recently presented to the Royal Society, London, a description of some very elaborate experiments made by him in explanation of the motions of small portions of camphor when thrown upon the surface of pure water. The following is an abstract of the conclusions he had deduced from his investigations:- That to succeed in the production of these movements the camphor must be thrown on the surface of clean water, in a perfectly clean vessel. That these phenomena may be also produced by certain salts, and other substances that diffuse readily over the surface of water. Thus the motions of camphor may be imitated by placing on water floating rafts of talc, tinfoil, paper, etc., smeared with or containing volatile oils, or any volatile liquid, such as ether, alcohol, chloroform, etc., provided there be a communication between such a liquid and the water. The camphor or other volatile substance, being slightly soluble in water, spreads a film over the surface of the water the moment that it comes in contact with it. The dimensions and form of this film evidently depend on those of the piece of camphor operated on, and in general the film separates more easily from broken surfaces and angles than from a smooth surface, as the broken surface of a crystal is more soluble than the natural surface. These films being constantly detached from the camphor so long as it is in contact with the

water, displace each other, the preceding film being conveyed away by the adhesion of the water in radial lines, which produce motion by reaction on the fragments of camphor, causing them to rotate in the same manner as a Barker's mill. These jets or films of camphor can be rendered visible by various means, as by fixing the camphor in water, and dusting the surface lightly with lycopodium powder, when a series of currents produced by the films will be made visible. The motions of the fragments of camphor on water are greatly influenced and complicated by their mutual attraction, and by the attraction of the sides of the vessel. The film of camphor diffused over the surface of the water is very volatile, disappearing as fast as it is formed, chiefly into the air, only a very small portion being retained by the water. Hence camphor wastes away much more quickly at the surface of the water than in water alone, or in air alone, because at the surface the film is being constantly formed at the expense of the camphor, and is spread out to the united action of air and water. Whatever interferes with evaporation lowers or arrests the motions of the camphor and the allied phenomena; so, on the contrary, whatever promotes evaporation exalts these phenomena. Effects which are displayed with great energy on a bright and sunny day, are produced either sluggishly or not at all on a wet, dull, or foggy one. A fixed oil forming a film on water will displace the camphor-film, and so permanently arrest the motions of the camphor; but a volatile oil will only arrest the motions while it is present and undergoing evaporation. The motions of camphor on the surface of water are increased by the action of the vapor of benzole and some other volatile substances, such vapors condensing in the liquid form on the camphor, and being then diffused by the adhesion of the water.

EXTENT OF THE EARTH'S ATMOSPHERE.

At the British Association, 1862, Prof. Challis presented a paper on the above subject, the object of which was to show that the earth's atmosphere is of limited extent, and reasons were adduced, in the absence of data for calculating the exact height, for concluding that it does not extend to the moon. It was argued, on the hypothesis of the atomic constitution of bodies, that the upward resultant of the molecular forces on any atom, since it decreases as the height increases, must eventually become just equal to the force of gravity, and that beyond the height at which this equality is satisfied there can be no more atoms, the atmosphere terminating in a small finite density. It has been generally stated that the earth's atmosphere is about forty-five miles high, but on no definite grounds, and the estimates of the height have been very various. Against the opinion that it extends as far as the moon, it was argued that, as the moon would, in that case, attach to itself a considerable portion by its gravitation, which would necessarily have some connection with the rest, there would be a continual drag on the portion more immediately surrounding the earth, and intermediately on the earth itself, which would, in some degree, retard the rotation on its axis. Hence, if, as there is reason to suppose, the rotation be strictly uniform, the earth's atmosphere cannot extend to the moon. The author also stated that if by balloon ascents the

barometer and thermometer were observed at two heights ascertained by observation, one considerably above the other, and both above the region in which the currents from the equator influence the temperature, data would be furnished by which an approximate determination of the height of the atmosphere might be attempted.

AUGMENTATION OF THE APPARENT DIAMETER OF A BODY BY ITS ATMOSPHERIC REFRACTION. BY PROF. CHALLIS.

For reasons given in the preceding communication, it was assumed that atmospheres generally have definite boundaries at which their densities have small but finite values. Two cases of refraction were considered in the one, the curvature of the course of a ray through the atmosphere was assumed to be always less than that of the globe it surrounds; and in the other, the curvature of the globe might be the greater. The former is known to be the case of the earth's atmosphere; and it was supposed that, à fortiori, this must be the case with respect to any atmosphere the moon may be supposed to have. On this supposition it was shown that the apparent diameter of the moon, as ascertained by measurement, would be greater than that inferred from the observation of an occultation of a star, because, by reason of the refraction of the atmosphere, the star would disappear and reappear when the line of vision was within the moon's apparent boundary. The same result would be obtained from a solar eclipse. It was stated that, by actual comparisons of the two kinds of determinations, such an excess to the amount of six to eight seconds was found. This difference may reasonably be attributed to the existence of a lunar atmosphere of very small magnitude and density. The author also stated that from this result there would be reason to expect, in a solar eclipse, that a slender band of the sun's disc immediately contiguous to the moon's border would be somewhat brighter than the other parts, and advised that especial attention should be directed to this point on the next occurrence of a solar eclipse. The case in which the curvature of the path of the ray is greater than that of the globe was assumed to be that of the sun's atmosphere; and it was shown, on this supposition, that all objects seen by the rays which come from the sun's periphery are brought by the refraction to the level of the boundary of the atmosphere, whether they proceeded from objects on the surface of the interior globe, or from clouds supposed to be suspended in the atmosphere. Accordingly, the contour of the sun should appear quite continuous, and the augmentation of apparent semi-diameter will be equal to the angle subtended at the earth by the whole height of the atmosphere. The apparent diameters of the planets will, for like reasons, be augmented to a certain amount by the effect of refraction; and, on account of the great distances of these bodies from the earth, the eclipse of a satellite will take place as soon as the visual ray is bent by the interposition of the atmosphere.

ATMOSPHERIC WAVES.

One of the most important results which have flowed from recent systematized meteorological observations has been the determination

of what is called the atmospheric wave, which means an ideal surface in the atmosphere at which the pressure is everywhere the same. If the atmosphere were still and undisturbed, this would be parallel to the earth's surface, and would never vary. Such, however, is by no means the case, the variation being sometimes enormous, rapid, and incessant, and strictly marking the conditions of calm and storm in the parts of the earth over which the wave is traced. One of the best illustrations of the action of this wave was afforded in the great storm which seriously injured the English and French fleets in the Black Sea on the fourteenth of November, 1854. This was by no means a local storm, as was proved by the comparison of more than two hundred and fifty reports from observers scattered all over the continent.

On the twelfth of November, 1854, the pressure of the air, which had been low, was enormously high on the line ranging from the west of England into France, reaching almost to the Pyrenees, but at various places east of this line the barometer was low. A great undulation of the air was taking place, and the ridge of a commencing wave was in the line here stated. As yet the storm had not commenced; but before four-and-twenty hours had elapsed this vast wave had moved toward the east, the north part of the crest having then reached Sweden, while the southern part had advanced far in the Mediterranean. It went through the great cities of Berlin, Dresden, and the southern part, and, having the Alps as its boundary, was lost in their windings. On each side of this crest the indications of storm were very marked.

Still another day elapsed, and the wave had now reached St. Petersburg and Dantzig, while its southern part was close to Vienna, and had entered the Adriatic, running down the coast of Dalmatia. On the fifteenth it was on the Carpathian Mountains, and on the sixteenth the crest had reached the Black Sea. Beyond that, there were no observatories to mark its progress. The storm took place when the low advancing wave glided over the gloomy waters of the Black Sea, long before the crest made its appearance. The weather is described as having been favorable enough until the fatal atmospheric wave bore down on the spot. Then, indeed, the barometer fell rapidly; but it was too late.

The high crest so curiously indicated could not be unaccompanied by depression. It commenced and was traceable at a great distance, and, in point of fact, the depression everywhere preceded the advancing wave, while another less considerable followed it. But while at the beginning the difference was small and the result unimportant, in proportion as the wave advanced toward the east, the hollow in advance became greatly deepened, or, in other words, the mercury stood very low indeed. The strength of the storm was felt where the depression reached its minimum, in the Black Sea, on the fourteenth of November. At that time the depression had been succeeded by the crest of the wave between St. Petersburg and the Dalmatian coast. The course of this storm, from its first commencement on the shores of the Atlantic till it reached the Black Sea, and the rate at which it was travelling, were matters perfectly within calculation after it had passed over the British Islands, and the time of