axis. By this construction, the telescope is rendered independent of the plane of the circle, and by the length of the axis is impelled to move in a truly vertical plane; and by reversing the axis, the line of collimation may be perfectly verified. This, therefore, renders it also a good instrument for observations in right ascension. The second novelty is the application of the two small levels or finders, which afford a very great convenience when repeating zenith distances; as by this application the telescope can be readily placed to those distances each time the instrument is reversed without the aid of a divided circle. There is also a novelty applied to the lantern which will be found extremely convenient. It consists of two plates of brass, having a square hole in each; these plates are moved in contrary directions by rack and pinion; and by this contrivance the observer is enabled to regulate the light in any proportion that may be required. There is also an entirely new application, which will be extremely advantageous when taking horizontal angles. This is the level which is applied to the principal horizontal circle, and which in every respect answers the purpose of a second telescope, while it is much more convenient, as the observer can instantly perceive the least possible motion of the circle without the necessity of changing his position; and if it should be required to take horizontal angles at night, the advantage will be very considerable. There is, lastly, a new appendage which will be found very useful when repeating the vertical angles. It consists of two arms fitted to the lower end of the centre that belongs to the horizontal circle, and has a motion sufficiently tight to keep it at the place to which it is set. When the telescope is presented to the object for observation, one of these arms is brought to coincide with a projecting piece in the triangular frame, and when the instrument is turned half round by bringing the other or opposite arm to coincide with the same projecting piece, the object will be again in the field of view of the telescope. In the third memoir, Mr. Francis Baily details "A Method of fixing a Transit Instrument exactly in the Meridian." This zealous and distinguished mathematician and astronomer, recommends, that when the transit instrument is placed nearly in the plane of the meridian, its accurate adjustment should be completed by observing the culmination of any two stars differing from each other considerably in declination. By this method, the necessity of having a building constructed, so as to command an uninterrupted view of the meridian from the northern to the southern horizon is avoided, since it may be successfully practised with portable instruments placed on the inner side of a window having a range of above 70° in altitude, or on the outer side, where they may be directed even to the zenith. "The stars which should be chosen for the purpose," Mr. Baily says, are those which differ at least 50 degrees from each other in declination, but the nearer that difference approaches to 90 degrees, the more correct will be the results. Their right ascensions, on the contrary, must be as near as possible to each other, a circumstance which will moreover prevent the possibility of any error arising from a variation in the rate of the clock during the interval of the observations." Passing over the mode of computing the useful table of declinations with which the paper concludes, we shall copy one of the two examples of its use and application, and of the mode of operating in such cases: "On July 1, 1819, I placed my transit instrument nearly in the meridian; and in order to ascertain how much it deviated from the true meridian, I observed the two stars y Lyra and Sagittarii. The passage of the former was observed at 18h.52′.37′′,3, and of the latter at 18h. 56′. 4",5 siderial time. The apparent right ascensions of those stars on that day were 18h.52′.9′′,8, and 18h. 55.39",7 respectively; and their declinations were 32°. 27′ N. and 27°. 55′ S; consequently the operation will stand thus 3. 29,9 d R) dᎢ = 3.32,6 dR = whence (d T 2",7. This value being negative, shows that the deviation is to the west: and in order to determine the quantity of the deviation, we must take the sum of the declinations (or the difference of the polar distances) of the two stars, which in this case is equal to 60°. 22′; or for the sake of round numbers, equal to 60°; and the declination of N (or the northern star) is about 32°. Consequently against the number 60, and under the column headed 32°, we shall find 1.39; which, being multiplied by 2",7, will give - 3",75 for the deviation of the instrument in time; and this multiplied by 15 will give 56",3 for the deviation in arc westerly." The importance of micrometers in the practice of astronomical observation is so great, that their improvement has constituted an object of continual interest to the philosophical artist. From this uninterrupted attention, numerous suggestions have arisen; and the Rev. William Pearson, by his extensive investigations, contained in the fourth, fifth, and sixth memoirs, has contributed in a high degree to the advancement of this valuable appendage to the telescope. To detail a method of measuring small angles that has for its basis that singular property of several crystallized bodies, double refraction, is the purpose of the first of these essays, entitled "On the Doubly-Refracting Property of Rock Crystal, considered as a Principle of Micrometrical Measurements when applied to a Telescope." The ingenious author candidly states, that the Abbé Rechon, about the year 1783, discovered, and first made known, a method of compounding two prisms of rock crystal in such a manner that any small object seen through them appeared double, and the constant angular distance thus formed was made the ground-work of a micrometrical telescope. Of this original instrument, not described in any English work, an account is given; and the improvements consequent upon the discoveries of Malus, Arago, and Lenoir, are successively noticed. But before the doubly-refracting prism can be rendered useful in measuring small angles, Dr. Pearson states that the constant angle which it measures, as viewed by the unassisted eye, must be accurately known; and also the magnifying power of the telescope as used with it; for on these data, the accuracy of the measure taken by this method entirely depends. The remainder of the memoir is accordingly devoted to a consideration of these two necessary objects. As a specimen of the manner of conducting these investigations, the second method of determining the constant angles of the prisms may be transcribed. "The prisms were now applied in succession to the small cap at the eye end of the telescope of 45.75 inches, with the view of measuring the distance between the centres of the same disc that had been used with the prisms in the cap of the object end. In the first position, all the three spider's lines were doubled; viz. the horizontal one and the two vertical ones. But turning the cap which held the prism round a little, brought the two images of the horizontal line into one, while it opened the other images or lines wider apart: a little motion given to the screw, however, soon brought the second and third lines into one strong black line, and left the first and fourth more faint, at equal distances to the right and left. In this situation I found I had obtained the measure of the angle wanted; for the second line of the first image was become coincident with the first line of the second image; and the distance of either of the extreme lines from the strong black one in the middle was the quantity of the measured angle, as indicated by the micrometer. The same thing was done at the other side of the micrometer's zero, and a mean of the two measures gave the true one without any index error. This process is as simple as accurate. When any prism is screwed into its place, the two images of the horizontal line must first be brought into one strong line, and then the two or four images of the coincident or separated lines (as the case may be) must be brought nicely into three, of which the middle one will be always much darker than either of the others by reason of there being then two images occupying the place of one. It is indeed astonishing with what degree of precision the small angle of any prism may be taken in this way; and what at first was not suspected, the micrometer indicated the same quantity, to whichever telescope it was thus applied with any prism, or even when it was detached from the telescopes altogether." Respecting the determination of the magnifying power of the telescope, the second of the objects before alluded to, the following quotation may suffice: "It has already been said, that if the constant angle of any prism be divided by the power of the telescope to which it is applied as a double image micrometer at the eye end, the quotient will be the measure of the angle, subtended by a line joining the centres of the two images of the objects observed. Therefore if the natural constant angle of any prism be divided by the measure obtained with any given power, the quotient will be that power, the constant angle being a quantity always equal to the product of any power by its corresponding measure. (To be continued.) ARTICLE X. Proceedings of Philosophical Societies. ROYAL SOCIETY. On the ultimate Analysis of Animal and Vegetable Substances, by Andrew Ure, MD. FRS. On the Analysis of Sea Water, by Alexander Marcet, MD. FRS. In this paper, the whole of which was not read, Dr. Marcet shows that the waters of the ocean do not contain mercury, as has been supposed, and that muriate of ammonia is a constant ingredient. ARTICLE XI. SCIENTIFIC INTELLIGENCE, AND NOTICES OF SUBJECTS CONNECTED WITH SCIENCE. I. Hydriodide of Carbon. In the Philosophical Transactions for 1821, Mr. Faraday described a compound of iodine and olefiant gas, but he had not at that time the means of ascertaining its composition. Since that period he has obtained it in greater quantity, and analyzed it. Four grains were passed in vapour over heated copper in a green glass tube; iodide of copper was formed, and pure olefiant gas evolved, which amounted to 1.37 cubic inch. As 100 c. i. of olefiant gas weigh about 30 15 grs. 1.37 c. i. will weigh 0413 gr. Now 4 grs. 0413 leave 3.587 iodine, and 3.587: 0·413 :: 117·75: 13.55 nearly. Now 13:55 is so nearly the number of 2 atoms of olefiant gas that, according to Mr. Faraday, the substance may be considered as composed of 1 atom of iodine... 2 atoms of olefiant gas. - 117.75 and is, therefore, analogous in its constitution to the compound of chlorine and olefiant gas, sometimes called chloric ether.-(Institution Journal.) II. General Return of Copper raised in Great Britain and Ireland in Ono Year ending June 30, 1822. Produce of Copper Ore in Cornwall in Six Months ending June 30, Average price of copper, 108/. 15s. per ton. III. Attraction of Moisture by Peroxide of Copper. According to M. Berzelius, this oxide attracts the humidity of the atmosphere very rapidly: it is reduced so readily in hydrogen gas that if a piece be strongly heated, but not to redness, and put into a bottle of the gas, the oxide takes fire, and is reduced, and water trickles down the sides of the vessel. According to the weight lost in this mode of reduction, peroxide of copper appears to be composed of Copper. 100.0 (Annales de Chimie). IV. Influence of Green Fruits upon the Air. M. Theodore de Saussure has given the following as the results of his experiments on this subject: Green fruits have the same influence as leaves upon the air both in sunshine and darkness; their action differs only in intensity, which is greatest in the leaves. During the night they cause the oxygen of their atmosphere to disappear, and they replace it by carbonic acid |