The experiment of M. Bravais, with a conical pendulum, (L'Institut, No. 920, Aug. 20, 1851,) is extremely interesting, and appears to have been entirely successful. He concluded from the nature of the case, that a free pendulum revolving in a circle would perform its gyrations in less time when its motion coincided with the rotary motion of the earth, than when it was contrary to it.

To settle this by experiment, he adopted two distinct methods -that of direct observation, and that of coincidences.

In the first method he observed the time occupied by the same pendulum in making from 900 to 1200 gyrations to the left, and then the duration of the same number of revolutions to the right, the pendulum being 33 feet 5 inches in length.

The observed times of rotation were as follows.

The second method was even more precise. Two pendulums, one slightly longer than the other, were suspended near each other, in the plane of the meridian. One was set revolving to the right, the other to the left simultaneously, and the passage of the threads across the meridian was observed through a telescope, whose optic axis cut the two threads when at rest. Let n and n' denote the number of oscillations of the respective pendulums between two successive coincidencs, n'=n+1, n' being the shorter. At the end of the series the pendulums were set revolving in opposite directions respectively, and the recurrence of the coincidences under the new circumstances, determines the numbers N and N'-N+1. Let the latitude, T the length of a sidereal day, t the time of a conical oscillation of the long pendulum, separated from the effect of the earth's rotation, and t' the same for the short pendulum. Then we shall have by theory, sin 2 1 S 1

One trial gave

Another

T

=

t+t' (n+n' N+N'

n=207·86; N=217.82

From these numbers the difference in time between a right and left oscillation is found to be, from the first trial, 0s 000725; from the second, 0·000710.

From these observations M. Bravais concludes that a seconds pendulum, revolving conically, would lose three seconds per day when moving from right to left, and gain the same when moving in the contrary direction. A pendulum 33 feet long would be retarded or accelerated 11-4 per day. From these observations M. Bravais also computed the length of a simple seconds pendulum, which he made 39in-1255 instead of 39in-1291 the number commonly adopted, (993mm-77, instead of 993mm-86.)

ART. XLIII-Extracts from the Proceedings of the Twenty-first Meeting of the British Association, held at Ipswich, July 2.*

1. On preparing Speculums for Telescope; by the Earl of Rosse. THE Earl of Rosse said that, having observed by the public prints that the President of the Association had, in his inaugural address, done him the honor of alluding with approbation to the attempts which he had lately made, and with considerable success, to produce plane spec ulums of silver for reflecting telescopes, he thought it might be ac ceptable to the Section to receive a brief account of them, and he had therefore come down from London that morning to give them such an account. In order to help their conception of what he had to say, he drew with chalk the annexed sketch. A, the great concave speculum at the bottom of the telescope tube, collecting the rays of light which came from a radiant point placed at a great distance beyond the A mouth of the tube to its con

B

jugate focus F. These rays were then intercepted by the small mirror B, and by it reflected to the focus F towards the side of the tube where they were received by the eye-piece of the telescope. Now, speaking roughly, it was found that about one-third of all the light which fell upon the great speculum A, was lost in the act of reflection, and that of the remainder a considerable portion, but not quite one-third, was lost by the second reflexion at B. Of course, since quantity of light is of the utmost importance to the perform. ance of these tubes, this rendered it a matter of the utmost import ance either to get rid altogether of the second reflexion at B, or to obtain some means of producing it subject to less loss of light than that caused by speculum metal; and accordingly, Sir William Her schel, the first who had succeeded in producing reflecting telescopes on a large scale, had, by inclining the great mirror slightly, brought the

* From the Athenæum of July 12 and 19, Nos. 1237, 1238.

direct image F to the side of the tube into such a position that it could be received directly in the eye-piece. But it was found that correct images could not be produced in this oblique position by speculums of the ordinary form, and as yet a means of grinding them to a form that would give correct oblique images had not been successfully devised.

He had received numerous and some most ingenious suggestions for placing the observer in such a position as to view the image F directly at the middle of the tube. One difficulty to be got rid of was, that the head and person of the observer would itself abstract much of the light that should be permitted to proceed to the speculum to be there reflected to form the image. Another was, that the temperature of the body of the observer, so much higher as it was than the surrounding air, tended to produce ascending currents, which both produced a wavering motion and an incorrectness of the image quite fatal to the accurate performance of the instrument. These could, perhaps, be guarded against, and he had received many most curious and some very ingenious plans for disposing even of the entire body of the observer by lying along in a tube properly and well-ventilated with means for conveying away the breath and heated air out of the telescope tube without interfering with the air in the tube itself. But one element in the problem seemed most strangely to have been overlooked by all, and yet he was bold to say it was the most important of any; he alluded to the diffraction caused by the head of the observer, or by the box or case in which it was proposed to encase it or himself. The effect of this diffraction upon the performance of the instrument would become more injurious also the larger the profile of the object which stopped off the light. Under all these difficulties, he had come to the conclusion that all attempts at viewing the direct image must be abandoned. He then turned his attention to prismatic reflexion, in which comparatively much less light was lost. But he found it impossible to obtain large and sufficiently well-formed prisms to insure the best action. But having obtained a very fine prism of a small size by a distinguished optician, he determined to try it. This required the prism to be placed much nearer to the apex of the cone of light reflected from the great speculum (F), and then the eye-piece required to be of such a form as greatly to diminish the field of view, a matter of the most serious import in the researches of the nebulæ in which he was engaged. This difficulty, however, he had surmounted by first using the larger reflector (B) to examine an extensive field of view, though less perfectly, and then, by a simple contrivance, turn it aside out of the cone of light, and more minutely examine the limited field of view afforded by the prismatic reflexion. Yet after all, he found this most injurious to the work of making accurate drawings of the appearances. But another still more serious difficulty is fatal to this mode of reflecting, namely, that although guided by the research of M. Jamin, and using the formulæ afforded by Cauchy, and every other formula he could conceive or arrive at the knowledge of, both as to polarized light and aberration, he had altogether failed in making this eye-piece achro matic; and if any mathematician of the Section could supply him with the means of securing this result, he would feel himself under great obligations to him, and would cheerfully furnish him with the precise

data of the problem. Under these circumstances, he perceived that there was no resource but to improve metallic plane reflectors as much as possible. Now, it was well known that in reflexion by silver much less light was lost than by any of the other metals, but, unfortunately, this metal was so soft, that great difficulties presented themselves in giving it the requisite degree of high polish. He had tried by the elec trotype process to procure a surface with a high polish by depositing silver on a surface of speculum metal, and treating it by the same process as that used by the distinguished officers engaged in the Trigono. metrical Survey of Ireland. But, unfortunately, he soon found that, use what precaution he would, either there took place an adherence of the deposited silver to the surface on which it had been deposited, or the polish was rendered imperfect by the means resorted to to prevent this. He tried copper similarly, which did not adhere, but produced a high degree of polish,-but of course its color and other properties rendered it inadmissible as a reflector. He then determined to endeav or to grind and polish a plane surface of silver, the softness of that metal having heretofore, however, caused the attempt to fail in the hands of the most experienced who had tried it. The processes of grinding and polishing are essentially different: in grinding, the substance, whether emery or other powder, must run loose between the substance which is used to rub it against the other and that which is to be ground; and he soon found that he could not use emery or any other grinding powder for bringing a surface of silver to a correct form, -for, from the softness of the metal and the unequal hardness of its parts, the emery was found to confine its action to the softer parts, leaving the harder portions in elevated ridges and prominences, something in the way that the iron handle of a pump which has been long and much used may be observed to be worn away. Hard steel he found he could bring to a very true surface, and even impart to it a high degree of polish; but the quantity of light it was capable of reflecting was by no means sufficient, nor could he succeed in imparting to the surface of silver by compression with highly-polished steel surfaces the evenly and highly-polished surface requisite for his purpose. At length, he found that he could by the use of good German hones grind surfaces of silver perfectly true, and he had now no doubt that he could with safety recommend for that purpose as the best material the blue variety of the German hone.

The next point was, to polish the surface to a true optical plane reflecting surface. This was by no means so easy a task as may be supposed-for although our eminent silversmiths do produce surfaces of silver of an extremely brilliant polish, as in the magnificent plateaux and other articles which they turn out, yet if any one will take the trouble to examine these surfaces, they will be found to be so irregular though highly polished, as to be entirely unfit for producing correct images by reflexion. And it is a singular fact that, although in the first part of the process of polishing, chamois leather of the finest kind was used to rub the rouge on the silver surface, yet the finer finishing pol ish had always to be communicated by the human hand. Nor would the hand of every individual answer :-the manufacturer had to select those with the very softest and finest grain, nor would the hand of

perhaps one in every twenty of the persons employed answer for thus giving the final finish. But it was obvious that the irregular action of the human hand would by no means answer the end he had in view. Suffice it to say, that at length, after many fruitless trials, he had succeeded in producing a polishing surface which seemed fully to answer the purpose by exposing spirits of turpentine to the continued action of air, or by dissolving a proper quantity of resin in the spirits of turpentine, and by means of this varnish applying the rouge to the same description of polishing substance which he used in polishing the speculum metal, and which he had heretofore so frequently described. By the use of this polishing substance he had produced a plane surface of silver which, as far as the photometric means he had within his reach would enable him to measure the light before and after reflexion, did not lose in that action seven parts of the hundred, and which, tested in the manner which he usually adopted, defined admirably.

The Astronomer Royal begged to know how Lord Rosse secured the plane form of the surface in grinding and polishing. The Earl of Rosse replied that, as to the mode of grinding, it was that commonly adopted for producing accurately flat surfaces. But the mode in which he tested it was peculiar. It was this:-a watch dial was placed before a good telescope, and as soon as the eye-piece was accurately adjusted to the position of most distinct vision, the plane mirror was placed in front of it at an angle of 45°, and the watch-dial was moved round by a simple contrivance to such a position as that its image should very nearly occupy the place it had been just removed from. If now the adjustment of the telescope for distinct vision remained unchanged, the proper form had been attained; but, if by drawing out the eye-piece more distinct vision was obtained, it was concluded it had received a convex form,-if on pushing it farther in it gave the image more distinct, then it was concluded the mirror had received a concave form. Prof. Chevallier wished to know whether Lord Rosse intended to form the great speculum similarly of silver. The Earl of Rosse replied that he had at present no expectation of doing so. That it was

a very different matter to grind and polish a speculum of a few square inches surface-which could be done by a small machine worked by hand, and from which to the eye-piece the light had to travel but about three feet to executing the same operations over the surface of a speculum six feet in diameter, and from which the light after reflexion had to travel a distance of fifty-three feet. For Newton, with his usual sagacity, had long since shown that any error in the form of the object speculum of a reflector was a much more serious injury to the performance of the instrument than an equal error would be in the plane speculum,and that for the identical reason he had just pointed out.

2. On a New Method of determining the quantity of Hygrometric Moisture in the Air; by Dr. ANDREWS.

In the absence of Dr. Andrews, Prof. Stevelly made this communication. Dr. Andrews had found on trial that several powders when well dried, would rapidly, effectually and completely take up the moisture of damp air passed through them, as effctually as the fused chlorid of calcium, which is too troublesome in the making, preserving