The surfaces he used were of glass; each was attached to a disc of cork, so as to render the whole of one mass or piece, of a specific gravity very little greater than that of the water in which they were to be immersed, and the prisms formed in this way were each accurately of the same size, weight, form, and surface; they were suspended by silk threads so as to form two pendulums, the weight of each being in the water 1 gramme (15.44 gr.). The thickness of each pendulum was 2 centimetres (.787 of inch), and the surfaces of glass opposed to each other, 10 centimetres (3.9371 inches) long, and half that height The length of the suspension threads was 18 centimetres (7 1468 inches). It is not our object here, however, to describe the apparatus, but simply to give a view of the author's expectation, and generally of the results he obtained. Operating with his apparatus, he found that when the surfaces were at any greater distance from each other at the commencement of an oscillation than 24 millimetres (.0984 of inch,) the time required for the first half oscillation was the same, amounting to 7 seconds. Hence it followed, that at this distance the two liquid layers on the moistened surfaces did not penetrate each other, and that therefore the thickness of this layer must be less than of a millimetre (.0492 of inch). As, however, the object in view was general confirmation rather than particular estimates of the strength of action, this point was not pursued, and M. Girard went at once to distances so small as to be within the limit of interference. These distances were five in number, measured by the diameters of five different wires used; the smallest being 563, the second .1127, the third .1579, the fourth .1917, and the largest .2481 of a millimetre in diameter. Then the gravity of the pendulums exerted in a direction opposite to their attraction, or in a horizontal line, was varied by moving the points of suspension to different equal distances on both sides, from the points at which the pendulums would have hung when just in contact. The following is part of a table expressing the results. Nos. 1, 2, 3, 4, and 5, are the wires, or the distances between the surfaces at the commencement of the oscillation. The figures in the first five columns express the gravitating power, in grammes, opposed to the attraction, and tending to separate the surfaces; and the figures in the last five columns express the time, in seconds, required in the corresponding experiments to complete the first half oscillation. No. 1. No. 2. No. 3. No. 4. No. 5. No. 1.No. 2. No. 3. No. 4. No. 5. 0.02763 0.02747 0.02740 0.027250.02710 0.05544 0.05532 0.05520 0.05510 0.05495 0.08386 0.08319 0.08318 0.08313 0.08286 0.11164 0.11154 0.11136 0.11133 0.11416 0.14023 0.14022 0.13980 0.13972 0.13956 832" 585" 380′′ 278" 163′′| 51 37 In looking down the last five columns, it will be readily observed how the time diminishes as the separating force of the pendulums (expressed in the first five columns) increase, the original interval between the surfaces being the same; this is a natural conse quence of known mechanical laws. But in looking along them horizontally, it will be observed how rapidly the times of the oscillations diminish as the original interval between the surfaces, as measured by the intervening wires, is increased, although the separating power is but very little altered. Without following M. Girard further, we may at once quote his conclusion, and refer our readers for further details to the memoir itself, in vol. xxix. of the Annales de Chimie. "Those surfaces which, being entirely immersed in a fluid, are susceptible of being wetted by it, when brought sufficiently near to each other to allow of the mutual penetration of their moistening layers, exert an attraction on each other by means of the interposed liquid, at distances which are sensible and capable of accurate measurement, and which increases as the distance is diminished." It appears that attention to temperature is of great importance in these kind of experiments. M. Girard found that other circumstances being equal, a variation of temperature from 5° to 20° centigrade (40° to 68° F.) caused a variation in the time from 783" to 520" only. 8. On an unnoticed Mechanical Principle-Explanation of the cutting of Steel by Iron.-The observation made by Mr. Barnes, in America, of the action of soft iron upon steel, has called forth the experimental remarks of many persons, and numerous explications of it have been given, none of which, however, have been satisfactory to all persons. The following is an abstract of parts of a paper by M. Allou: it has been inserted in the Bibliothèque Universelle, xxix. p. 192. It is remarked, that if the disc of iron be moved with a velocity continually increasing, it is at first acted upon by the steel plate or piece presented to it; with a certain velocity, no action either way takes place; and with a still greater velocity, the steel is cut by the iron. In explanation of this fact, M. Allou states, that if two bodies equally hard and elastic, such as two balls of ivory, strike each other with velocities nearly equal, each of these will experience a similar blow, and the change of form which results is instantly corrected by the elasticity of the bodies. But if the first is moved with a velocity incomparably greater than the second, and if the latter, though harder than the former, is suscep tible of being cut or traversed, it will suffer either the one or the other effect, without receiving any sensible velocity, all the action impressed upon it being employed in piercing or penetrating the ball or body struck. In support of this principle are cited various well-known effects, amongst which are, the penetration of an open door by a candle shot from a gun, no motion being communicated to the door; the legerdemain trick of breaking a stick resting on the edges of two glasses filled with water, by striking it sharply in the middle with another stick, the glasses not being moved or the water spilt; the cutting off of twigs and flowers in a garden or the fields, by a sharp blow on the stems with a light switch; the separation of a snake into portions when struck with a hazeltree rod; the mode of blasting introduced by the engineer Jessop, in which, after the introduction of the powder into the hole, the upper part is filled merely by sand: in all these cases M. Allou considers the effect as due to the want of reaction in the passive body, as regards the attack made upon it by the active body. "All bodies resist motion," says M. Francœur, in his Treatise on Mechanics," and it is in resisting that they receive it. Those which are instantaneously pierced or destroyed, not having offered appreciable resistance, should not therefore receive motion, and this is confirmed by experiment." Speaking of the branches, flowers, and the snake, cut in pieces by the switch," the shock is so rapid and unexpected, that the muscular fibres of the reptile, as well as those which form the tissue of the flowers and branches, have not time to react. Now it is this want of reaction which constitutes the phenomenon;" and in the experiment of Mr. Barnes," is it not natural to think that the shock on the steel by the cutting edge of the disc is so sudden and unexpected, that the molecules of the first have not time to react on those of the latter, and are thus rapidly removed at each contact." Other facts of a similar nature with the former are then cited in support of this view of the matter, but they add no new proof. The author thinks that the mechanical principle of which the phenomena described are the effects, has not had sufficient importance attached to it. We may be permitted to observe that we think if any thing else was intended in the explication than was previously well known, it has been left in too obscure a state to persuade us of its importance, or indeed of its accuracy, It seems difficult to comprehend how, when two similar bodies meet, they should possess different properties dependent merely upon the superior velocity of one over that of the other; and taking the cases of the two ivory balls, we doubt whether any difference would be observed in the effects occasioned by their coming together with a certain great velocity, whether the one or the other ball had all the velocity, or whether each had half. Again, if in all the cases quoted, including the curious experiment of Mr. Barnes, of cutting steel by iron, things could be changed, so that the body in the experiment actually in motion could be at rest, and the body previously at rest could have the motion transferred to it, would not the effects be the same? If in place of making the iron disc revolve and holding a steel file to it, the disc were at rest and the steel file revolving with equal rapidity round it, the same part being always in contact with the disc, is there any reason to expect a different result than that now obtained? We think not; and, indeed, going at once to the principle sought to be established, we cannot think that action and reaction are distinguished from each other by any difference in the times which they respectfully require, or that where there is time for the one there is not time for the other. 9. Magnetic Rotation.-M. Arago's beautiful experiment is now well known, and, as it deserves, attracts attention every where The following are some results obtained by MM. Prevost and Colladon, which, as they vary slightly in certain points from those as yet published in this country, will be interesting to such as pursue this branch of science. A disc formed of a thick copper wire rolled in a spiral, produced much less effect than a perfect disc of the metal of the same weight and size. A disc of glass covered with lead, or a single leaf of tin glued on to wood, sensibly deviated the needle. Wood alone, or sulphur, or a disc of peroxide of iron, had no appreciable effect. A disc of hammered copper deviated the needle more strongly than the same disc annealed. A screen of copper, or copper and zinc interposed, diminished the effect without destroying it. The diminution was greater as the screen was thicker, or placed nearer to the needle. A screen of glass had no influence. If the interposed metallic screen were pierced by an aperture equal in diameter to the length of the needle, its effect was very nearly the same. A vertical magnet suspended in the centre of a cylinder of copper remained unmoved, whatever the direction or rapidity of rotation of the ring. When two needles were fixed together in a similar direction, the effect increased; when they were placed with their opposite poles together, it ceased entirely. A needle magnetized, so as to have similar poles at its two extremities, was the apparatus most sensible to the motion of the discs. It was one of this kind which the authors used in their delicate experiments. The conclusion arrived at by MM. Prevost and Colladon is, that the effects are due to a transient magnetization of the discs, which, not being able to modify itself with a rapidity proportional to that by which the different points of the disc are displaced by rotation, are transported to a small angular distance from the needle before they are changed, and draw it after them. This is the same explanation in effect as that of MM. Herschel and Babbage. Experiments made with care to determine the influence of the velocity and the distance of the discs, indicated that the angles of deviation, and not their sines, augmented proportionally with the velocity, at least, within certain limits, and that the sines of the angles of deviation increased in an inverse ratio of the power 2 of the distance. They were careful to employ, in this determination, discs having diameters very great in comparison to the length of the needle.-Bib. Univ. xxix. 316. 10. On the Formation of a Society for the Cultivation of Naval Architecture. It is remarkable in a country which owes its opulence to commerce, and its political power to its naval superiority, that no association among the learned should exist, for the purpose of cultivating, in an especial manner, the important subject of naval architecture; and the circumstance is the more to be wondered at, from the establishment of philosophical societies forming so striking and peculiar a characteristic of the age. There are so many advantages to be derived from the co-operation of numbers in the prosecution of scientific pursuits,-advantages first pointed out by the prophetic mind of Lord Bacon, and which experience has in every way confirmed, that it would be regarded as an unnecessary waste of words, to attempt, at the present time, to advance any new arguments in favour of a system, which has tended in so splendid and triumphant a way, to ennoble and dignify the exertions of man. At the present moment, besides the Royal Society, which may be said to embrace the whole range of the physical sciences, we have societies devoted to the exclusive cultivation of particular departments of natural knowledge, such as the Geological Society, the Astronomical Society, &c. Now there is perhaps no subject which requires more essentially the aid and co-operation of numbers than naval architecture, involving as it does, so extensive a field for inquiry, and so beset as all its elements are with difficulties of so peculiar and intricate a kind. At the present moment there is a spirit of inquiry abroad respecting ship-building, which no antecedent period ever exhibited; and which, if taken at the flood, and before the causes that have awakened it again subside, must produce consequences of a very important kind. What seems to be wanting, is a sort of focus, or common point of union, to rally the disjointed and insulated speculations now afloat respecting it, and to concentrate the efforts of those who feel interested in its advancement. This might be most readily and effectually done, by instituting a society; the object of which should be, to encourage theoretical and experimental inquiries, |