tain telescope makers by means of hand labour. His lordship attempted no such imperfect process: he invented a grinding and polishing machine, by which, after repeated trials, he realised the means of making perfect specula of any dimensions, from one to six feet in diameter. A difficulty not less formidable impeded his operations-the casting of a speculum of sufficient size and strength. Herschel discovered Uranus with a speculum of forty-eight inches diameter; but it became tarnished, from defects of composition, and was abandoned for one of eighteen inches. That which was now desirable was a nice adjustment of metallic compounds, which, while affording a durable lustre, would also give that degree of ductility by which the speculum could be handled and, ground without liability to fracture. After numerous trials, it was found that the best combinations were of tin and copper, in the proportions of rather more than two of copper to one of tin; or, more correctly, 126:4 parts of copper to 58.9 of tin. These proportions fused together, and cast in a mould, made a preferable speculum metal. With a knowledge of the proper proportions to be used, Lord Rosse commenced making a speculum which should be three feet in diameter, by casting sixteen separate portions, to be soldered together afterwards. After repeated trials, he made one of this compound kind; and it was by the experience he acquired in doing so, that he became acquainted with the method of casting a large speculum in a single piece. Several tormenting difficulties attended his first efforts. Small air-holes were formed in the metal, and the speculum cracked in cooling. A mould of sand, and subsequently a mould of cast-iron, failed in giving freedom from pores. The desideratum was a kind of mould which should retain the molten metal, and yet allow the airglobules to escape. Such was at length discovered, and it is this which has deservedly stamped Lord Rosse's name with celebrity, reducing as it does the casting of specula to a certainty. The simplicity of the contrivance causes it to appear a matter of no great wonder; but, like the plan pursued by Columbus to make the egg stand on end, it is easy only when it is known. The contrivance consisted in making the bottom of the mould of layers of hoop iron, bound closely together, with the edges uppermost. By this means the iron conducted the heat away through the bottom, so as to cool the metal towards the top, while the interstices between the hoops, though close enough to prevent the metal from running out, were sufficiently open to allow the air to escape. At my visit, I had an opportunity of seeing this singular mould; it was a large disk of malleable iron, the layers of which were about half an inch thick, and to all appearance so closely welded, that water could not filter through them. The first large speculum thus made in a single piece was a round plate of metal three feet in diameter, nine inches thick, and upwards of a ton in weight. On becoming solid, it was removed, to be annealed, to a brick oven, the mouth of which is level with the ground, at the distance of a few feet. The oven was nearly red-hot when the speculum was shut up within it, and from this temperature it was allowed to become gradually cool, when the annealing was completed. The time required for annealing a plate of this large size is, I believe, about three weeks; and yet, with all this attention, so brittle is the metal, from sudden variations of temperature, that a warm hand laid upon it in a cold night will make it fly in pieces. - Following the mass to the next stage in its progress, we find it placed, with the face upwards, upon a turning apparatus. Here it is seen moving round slowly, immersed partially in water, in order to be kept cool, while a grinding or rubbing tool works on its surface. By means of this grinder, with emery and water, and the adjusted rotative motions, the proper parabolic curve, along with a certain degree of smoothness, is produced; after which the surface is similarly polished with resin, and some other substances. The required curve is ascertained in the following manner: The grinding is performed on the ground-floor of a house, adjoining which is a tower several storeys high. On the top of this tower is erected a mast, the summit of which is ninety feet from the speculum on the grinding machine. To the top of the mast the dial-plate of a watch is fixed, forming a small round object relieved against the sky. When the workmen wish to try the capacity of the speculum, it is cleared of its grinder, trap-doors overhead are opened, the figures on the dial-plate are reflected on the speculum, and this reflection is seen at the regulated focal distance by means of a small eye-piece: in other words, a temporary telescope is formed without a tube; and by this ingenious yet simple device, the speculum is wrought to that nice parabolic figure which brings the incident rays to an exact focus. Of the extreme accuracy required, we may obtain some notion from a statement of Lord Rosse, that an error of a small fraction of a hair's-breadth would destroy all hope of correct action; and Dr Robinson mentions that the smallest inequality of local pressure during the polishing process, would be attended with the result of changing a well-defined star into a blot or comet. The speculum, nevertheless, was polished in the short space of six hours. The speculum, so fortunately completed, was fixed or bedded on three iron plates, which gave it support, and then transferred to its appointed situation in the tube. This, as I have already noticed, is three feet in diameter and twenty-six feet long, and attached to an apparatus on the lawn, by which it can be brought to bear on any point of the sky a short way above the horizon. The machinery for moving it round, and raising and depressing it, is simple and ingenious; and notwithstanding its size, it may be adjusted with the greatest ease. Two step-ladders form part of the apparatus, and by these we mount to a gallery, which can be raised or lowered to any required height. In order to procure an observation, the tube is first brought to bear on the star or other object, and the gallery being raised, we ascend to it by one of the ladders. On reaching the gallery, which is a small railed platform sufficient to hold several persons, we find ourselves close to the telescope, near its upper extremity; and here, on looking through a small eye-piece fixed to the tube, we at once recognise in the obliquely-placed mirror within the object of our observation. The tube is of wood, hooped with iron, and the focal distance of the speculum is twenty-seven feet. I was rather surprised to find that the mouth of the tube remained permanently open-the natural idea arising in my mind being, that the rain and vapours would enter thereby, and injure the speculum at the lower extremity. I was informed, however, that the telescope is lowered in wet weather, and that the speculum is confined in a case, the cover of which is withdrawn by an exterior action when required. A vessel of quicklime is also kept constantly in the case, for the purpose of absorbing the moisture and acid vapours by which the speculum might be tarnished. The power of the telescope depends on the glasses employed in the eye-piece. This requires a little explanation. The rays collected by the speculum are directed on the mirror at the proper focal distance, and there reflected clearly, or brought within telescopic reach. A telescope must be employed to magnify the image, or draw it out; and accordingly, a small telescope like a pocket prospect-glass, technically an eye-piece, is used for this purpose. But this eye-piece requires to be used with discretion; its glasses must be shifted according to circumstances. Unless the atmosphere be exceedingly clear and dry, a powerful telescope will magnify its particles, and these will seemingly form a haze interceptive of lucid observation. Different densities, from contending streams of warm and cold air, will have a similar result; and if the atmosphere be excessively cold, as in a Russian winter, floating spicule of ice, invisible to the naked eye, will be magnified so as equally to interrupt perfect astronomical observation. Such contingencies present serious drawbacks to the increase of power in telescopes; and in the instrument we have been describing, they are attempted to be overcome by employing various eye-pieces, whose magnifying powers range from 180 to 2000. The performances of this magnificent twenty-six-feet telescope were found to be far beyond those of any previously-constructed instrument. Certain patches of light or nebulous matter in the heavens were resolved into clusters of separate stars; stars hitherto seen but dimly, appeared round and well-defined; and on the surface of the moon, valleys, mountain-tops, and craters of volcanoes were plainly visible. Gratifying as were these re sults, Lord Rosse considered that something still grander could be achieved; and before the twenty-six-feet telescope was well finished, he projected the gigantic instrument the dimensions of which have been already given. The casting, grinding, polishing, and mounting of this monster speculum were pretty nearly a repetition, on a larger scale, of what had been previously done. When finished, the speculum was placed in a square box, which is attached to the lower end of the tube, and by means of a door, can be entered at pleasure. This box adds six feet to the length of the tube, which, like its predecessor, is of wood, hooped with iron like a barrel, and so wide, that a tall man could walk through it without stooping. It is this huge black funnel that I have spoken of as being suspended between high and strong walls of Gothic architecture. It swings with a clear space of twelve feet on each side; and so far it can be drawn aside, giving half an hour before and after the meridian. By means of a windlass, and a most skilful adjustment of chains and counterpoising weights, it can also be brought to the zenith, or turned fairly round from south to north, always within its bounds of twenty-four feet. Enormous as are its dimensions, and although weighing altogether twelve tons, it seemed to me about as easily moved as the other telescope; and it is as much in the mechanical contrivances for effecting this purpose, as in anything else, that the peculiar merit of the structure consists. At the period of my visit, few observations had been made by this monster instrument, some parts of whose apparatus, indeed, were not completed; but so far as a trial in favourable conditions of the atmosphere had been attempted, the results had been interesting and important. [The results since obtained are given in the subsequent sections of the present paper.] Unfortunately, the atmosphere, during the two nights which I attended in Lord Rosse's grounds, was not propitious for observation. I had an opportunity, for only a few minutes, of seeing a group of binary stars of different colours; and these certainly were distinct, clear, and lustrous, like a pair of glittering diamonds. The moon, on both the occasions of my visit, kept provokingly under a tract of clouds; and the hemisphere, except for a brief interval of time, was loaded with an Irish drizzle. Yet this was no solitary disappointment. Astronomers spend weeks at Parsonstown, and yet perhaps enjoy only one or two really good nights for observation. When to these atmospheric impediments is added the comparatively limited lateral range of the great telescope, a long space of time will appear to be required for making a thorough and general search of the heavens. Disappointing in some respects as may be the result of such transient observations as that which my time allowed me to make, not the less grateful will be the feelings of visitors towards Lord Rosse for his unexampled politeness and liberality in throwing open to them his workshops and the whole of the telescopic apparatus which adorn the beautiful grounds around his mansion. To use the complimentary language of a local writer-" With a rank and fortune, and every circumstance that usually unfit men for scientific pursuits, especially for their practical details, if his lordship only encouraged those undertakings in others, he would merit our praise; but when we see him, without losing sight of the duties of his station in society, give up so much time, and expend so much money on those pursuits himself, and render not only his name illustrious, but his rank more honourable, we must feel sympathy in his successes, and rejoice that he has obtained from all quarters the highest and most flattering encomiums, and that he can now enjoy, in the use of his telescope, the well-earned fruits of his previous labours." new. REVELATIONS OF THE INSTRUMENT. The telescope is in constant use for descrying distant objects on the earth, and in this capacity it very much extends the range of our knowledge of what is going on around us. But it has not, properly speaking, enabled us to make new discoveries on the earth, like the microscope, for we can examine close at hand everything that the terrestrial world contains on its surface. But by turning the instrument to the heavens, a vast number of appearances have been made known to us that were entirely Celestial bodies have become visible that we did not previously know to exist; the blank spaces of the sky have been filled with hosts of stars. The bodies, also, that were always visible, have been viewed with so much distinctness, that many new revelations have been made regarding their constitution. We shall, in detailing these various discoveries, describe in order the sun, the moon, the planets, the comets, and the starry universe-presuming that the reader is somewhat acquainted with the order of the solar system, and with that of the starsystems beyond. THE SUN. The sun had all along appeared to mankind as a broad, round face, covered with an even and uniform glare of light. But when the telescope, immediately after its invention, was applied to the solar body by Galileo, he found that it was not of a uniform brightness, but was speckled here and there with dark spots. The application of the most powerful telescopes of recent times, in addition to these spots, has discovered the whole surface of the sun to be very irregular and unequal in brightness. The light is made up of spatches of all different degrees of brilliancy, being a confused mixture of more and less bright portions; and the disk is mottled with dark dots or pores, that are perpetually changing and shifting, closing up and breaking out |