inexact. This caoutchouc presses upon small, perfectly capillary tubes with the point drawn out, penetrating 1 or 2 millims. into the tube; and this has the further advantage of causing the mercury to descend softly and without shocks, when it takes its successive positions of equilibrium. The construction of the instrument allows the tube to be easily replaced by the operator himself if it should be broken; it also allows it to be cleaned and dried easily, if this be necessary, for to avoid alterations of curvature in the menisci which terminate the column of mercury, the tube must be perfectly dry. We also describe a small instrument of easy construction, by means of which the mercury is always introduced into the tube in the same quantity. I shall indicate the formulæ giving the atmospheric pressure. Let I be the length of the column of mercury, a that of the glass tube, L the atmospheric pressure in parts of a metre. Let h be the distance from the upper surface of the mercury to the extremity S of the tube, after i operations have been effected. We will suppose that before commencing operations we have • h=h. Let L=nl, (a-1)=ml, h=ql, ho=qol. Perform the (i + 1)th operation, and suppose that q=q+Aq; we have the equation with finite differences We shall have to deduce from this equation n in function of q and i; qo being very small, we neglect it at first, and the application of the formula of Lagrange to the equation n1 =p+ + i+1 _ (i+1)(2i+1) (i+1)(2i+1) (5i+3) 6p3 24p3 n, is only the approximate value of n. Making +... ; In this way the question is solved, but it is more simple to employ the formula -i μπ to calculate tables which will at once give n,, in function of i and μ; this is what we have done in calculating the table relating to i=4, i=5, up to i=11, which is sufficient for all possible cases. As the formula is not exact, we have endeavoured to make the necessary correction to μ, so that on consulting our tables, we may come at once upon the exact value of n, and we have found The coefficient μo is obtained by an observation made simultaneously with the repeating and ordinary barometers. We have calculated a small table giving this correction Aμ. The accuracy of the repeating barometer to that of the ordinary barometer is in the proportion of q to n; taking a = 0·450 M, 1=0.070 M; this exactitude at the level of the sea is nearly as is to 1; that is to say, that 1 millimetre of variation of the ordinary barometer corresponds with about a millimetre of that of the repeating barometer. But this exactitude increases in proportion as we rise, because then n diminishes, and at an elevation of 2000 to 3000 metres the two exactitudes are nearly equal. In a numerous series of observations made with the two barometers, the disagreement rarely rose to 1 millim.; most frequently it was limited to a few tenths of a millim. more or less. Up to a certain point the observations may be verified by making them with two consecutive values of i, which ought to give nearly the same value for n. I must not omit to mention that my formulæ suppose the tube to be perfectly cylindrical, which is very rarely the case, but from their construction the tubes affect a slightly conical form. To remedy this inconvenience, I show that with a sufficient approximation it suffices to make the observations twice, keeping each extremity of the tube successively in the air, and taking the average of the values thus observed of 9 h μπ =-1° m a -Comptes Rendus, March 30, 1857, p. 658. ON A NEW LOCALITY FOR THE MINERAL ATACAMITE. This mineral (CuCl +3CuO HO) occurs abundantly crystallized on malachite, massive quartz and gozzan, obtained from the extraordinary deposit of malachite in the Serra do Bembe near Ambriz, on the west coast of Africa. It occurs in small, distinct, rhombic prisms, translucent, and of a light to opake and dark green colour with vitreous lustre ; the symbols, for the crystals being, according to Naumann's system, o P. P∞. = The probable existence of this mineral in considerable quantity at the above locality, as at Atacama and other parts of Chili, remains to be yet ascertained. THE IMPROVED INDUCTION COIL. To the Editors of the Philosophical Magazine and Journal. May 18, 1857. GENTLEMEN, I regret that Dr. Noad's disinterested conduct to a stranger should have induced Mr. Hearder to make such statements as those which appeared in the Philosophical Magazine of this month. He there intimates that Dr. Noad introduced the instrument of his friend during a lecture given at the Polytechnic Institution, thereby doing him (Mr. Hearder) a commercial injury. Now I had seen Dr. Noad but once before that lecture was delivered, I could not therefore have been honoured by his friendship; and had I been so fortunate, would it have been just to suppress my humble attempts at improvement for the sake of Mr. Hearder's pecuniary advantage? With regard to Mr. Hearder's claim to the honour of having introduced the improved induction coil into this country, I may state that the coil which was exhibited at the Polytechnic Institution on the 29th of September, was commenced in January and finished in March 1856. This is not of much importance; and I should not have named it, had it not been stated that I had constructed my coil but a few weeks antecedent to the day of the lecture; for it appears to me that the individual who first makes an improvement public ought to have some slight share of the credit for such improvement, although there may be others working in the same field with him. I fear that if Dr. Whitehouse comes forward as a claimant for the honour of improving the induction coil, we shall be poorly off; for he has not only improved that instrument, but practically applied it to telegraphic purposes, thereby rendering it of high commercial im. portance. I am, Gentlemen, Yours obediently, C. A. BENTLEY. ON THE ABSORPTION OF LIGHT IN TRAVERSING COMETS. BY M. BABINET. I have endeavoured by every means which can be furnished by optics, to ascertain the probable mass and density of comets. According to the estimates of Sir John Herschel, of Bessel, Struve, Admiral Smyth, and even of Arago, the contrast of intensities has given me as the atmospheric equivalent of a comet a number so small, that it reduces to almost nothing the mass and the density of these stars, which are not even gaseous, as is proved by the exact measurements of Struve and Bessel, who were unable to detect any refraction in the nucleus of comets. It will be easily supposed that the absorption of light in traversing material media was not the last optical notion to which I had recourse in order to investigate the very exceptional nature of these moveable masses of nebulous substance. But the result at which one arrives is so exorbitant, that I should not have ventured to place it before the Academy if it had been anything but the immediate deduction from facts and laws admitted by every one. Sir John Herschel, as far as I am aware, is the only person who has indicated how weak was the absorption of light in traversing comets, although nearly all other astronomers have perceived that the nebulosity of comets did not sensibly weaken the light of the smallest stars when these were seen through their tails, or even through their nuclei. The following are the numbers relating to the fourth comet of 1825, called the Great comet of the Bull (Hind's Catalogue), discovered by Pons, at Marseilles, on the 15th of July. It passed at the perihelium on the 10th of December; and at a moment when it could not yet have become hollow, namely the 15th of August, Pons ascertained that a star of the fifth magnitude, which was seen across its centre, had undergone no sensible diminution of brilliancy. From this we may conclude that the star had not lost half a magnitude, that is to say, a fifth of its light (if we admit, with Messrs. Johnson and Pogson, that a star diminishes one magnitude when its light is reduced to ths of what it was originally). The star consequently retained at least 4ths of its original brilliancy. We know also that the light of the stars in passing even perpendicularly through the atmosphere, loses more than 4th of its intensity, and is not more than 2ths of what it was at its entrance into the upper strata of the air. Taking 8 kilometres as the thickness of the whole atmospheric stratum reduced to the density possessed by this fluid at the surface of the earth, a single passage would reduce the light to ths of its original intensity; two similar passages would reduce it to ths of ths, or ths; and lastly, a passage through a space a thousand times greater would reduce the intensity to the number ths raised to the thousandth power. Now this is precisely the passage made on the 15th of August 1825, by the light of the star across the comet, the nucleus of which was more than 800 kilometres in diameter. This number ths raised to the thousandth power is a fraction having unity for its numerator, and for its denominator a number of 5 figures followed by 120 cyphers. To assimilate the comet to dilated atmospheric air, we must therefore take air so rare that its density, multiplied by the above immense number, would only be equal to the fraction. Let x be this hypothetical density, we should have 1 (4) 1000 =4. We get for r a fraction having unity for its numerator, and for its denominator a number superior to unity followed by 125 cyphers. When Herschel, in his last work on astronomy, spoke of a few ounces as the mass of the tail of a comet, he found nearly as many disbelievers as readers. Nevertheless his calculation is exaggerated in comparison with the preceding determination. But how are comets visible? I shall examine this question in accordance with my theory of the light which is disseminated by small corpuscles forming a non-continuous medium.-Comptes Rendus, May 4, 1857, p. 885. LONDON, EDINBURGH AND DUBLIN PHILOSOPHICAL MAGAZINE AND JOURNAL OF SCIENCE. SUPPLEMENT TO VOL. XIII. FOURTH SERIES. LXIII. On the Sounds produced by the Combustion of Gases in Tubes. By JOHN TYNDALL, F.R.S.* IN N the first volume of Nicholson's Journal, published in 1802, the sounds produced by the combustion of hydrogen in tubes are referred to as having been "made in Italy:" Dr. Higgins, in the same place, shows that he had discovered them in the year 1777, while observing the water formed in a glass vessel by the slow combustion of a slender stream of hydrogen. Chladni, in his 'Akustik,' published in 1802, page 74, speaks of their being mentioned, and incorrectly explained, by De Luc in his 'New Ideas on Meteorology:' I do not know the date of the volume. Chladni himself showed that the tones produced were the same as those of an open pipe of the same length as the tube which encompassed the flame. He also succeeded in obtaining a tone and its octave from the same tube, and in one case obtained the fifth of the octave. In a paper published in the Journal de Physique in 1802, G. De la Rive endeavoured to account for the sounds by referring them to the alternate contraction and expansion of aqueous vapour; basing his opinion upon a series of experiments of great beauty and ingenuity made with the bulbs of thermometers. In 1818 Mr. Faraday took up the subject†, and showed that the tones were pro duced when the glass tube was enveloped by an atmosphere higher in temperature than 212° Fahr. That they were not due to aqueous vapour, was further shown by the fact that they could be produced by the combustion of carbonic oxide. He referred the sounds to successive explosions produced by the periodic combination of the atmospheric oxygen with the issuing jet of hydrogen gas. This is undoubtedly the true source of the sounds. I am not aware that the dependence of the pitch of the note on the size of the flame has as yet been noticed. To this point I will, in the first place, briefly direct attention. *Communicated by the Author. † Journal of Science and the Arts, vol. v. p. 274. Phil. Mag. S. 4. No. 89. Suppl. Vol. 13. 2 K |