THE QUARTERLY JOURNAL OF SCIENCE. OCTOBER, 1868. I. DESCRIPTION OF THE GREAT SOUTHERN By WILLIAM CROOKES, F.R.S., &c. SEVERAL years ago the Government of Victoria voted the sum of 5,0007. for the construction of a large equatorial telescope to be erected at Melbourne, for the observation of the nebula and multiple stars of the Southern Hemisphere. The construction was entrusted to Mr. Grubb, F.R.S., of Dublin, who stands in the first rank as an optical and telescopic engineer in the manufacture of instruments in which every step is required to be preceded by mathematical research. At the commencement of the present year the telescope was completed and examined by the Committee of the Royal Society who had superintended the work throughout. In a report recently communicated to the Royal Society, the Committee express their unanimous opinion that the equatorial is a masterpiece of engineering. Before this notice meets the reader's eye, the telescope will probably be on its way to Australia, and as it is beyond comparison the largest and most elaborate equatorial ever constructed, it seems due both to the constructor and to the importance of the instrument that a detailed account of it should appear in the Quarterly Journal of Science.' Through the kindness of my friend Mr. Grubb, who has placed at my disposal drawings, photographs, and ample descriptions of all parts of the instrument, I have ventured to undertake this office. The great Melbourne telescope is of the form known as the Cassegrainian reflector, and is mounted equatorially on what Mr. Grubb calls "the German system improved." The mirrors, two being supplied in case of accident, are 4 feet in clear aperture, 4 inches thick, 30 ft. 6 in. in focus, and rest in VOL. V. 2 I their box on Mr. Grubb's system of hoops; the whole system of suspension and levers, presently to be described, weighs altogether nearly 2 tons. Of the tube, 7 feet is made of boiler-plate iron, quarter inch thick, to which is attached by flanges and bolts a skeleton tube, 21 feet long, of steel bars, 3 inches wide at bottom, 12 at top, and of an inch thick, wound spirally round rings of carefully turned angle iron and riveted at the joints, forming a spiral lattice of amazing strength, stiffness, and freedom from tremor. The 7 feet of boiler-plate tube weigh 1,300 lbs., and the 21 feet of the ventilated tube only 1,370 lbs. At the upper end of the tube, about 25 feet 6 inches from the large mirror, is bolted a very stiff hollow arm of steel-plate, on the extremity of which is a V-shaped gun-metal casting, in which slides an arm carrying the small mirror of 8 inches diameter. This arm is acted upon from behind by a screw, from a pulley on the shaft of which wire-cords are carried over iron guide-wheels down the side of the tube, where they are wound round a wheel to which motion can be given by the observer, for the purpose of focussing. The polar axis is made up of four distinct parts, viz. a cube 3 feet square, to which is bolted on one side a cone 8 feet long which terminates with a bearing 12 inches diameter, resting in a peculiar "plumber-block" on the polar pier, on the opposite side a short toe-piece, which carries on parts prepared for them the two hour circles, sector, and clamp, and terminates in a bearing 6 inches diameter resting in a Y block in the equatorial pier, and on a third side a bell-shaped casting about 2 feet long, which terminates in a slide carrying one bearing of the declination axis, the other being in the side of the cube opposite the bell. The declination axis is 24 inches diameter at the bearing next the telescope, and 12 inches at the other, the bearings being 5 feet asunder and the axis itself about 9 feet long. It carries at one end the telescope strapped into its cradle, and at the other counterpoise weights, amounting to over 2 tons. The counterpoise weights are four circular cast-iron boxes, consisting of a ring 10 inches diameter. which is bored out to fit the axis, and an outer ring 30 inches diameter, both 6 inches deep, connected by a plate to form a bottom, and divided into six segments by ribs. These chambers are mostly filled with lead, the outer one being left with a little spare space for adjustments. The bearings of the polar axis, on the principle of Ys, are constructed with as much delicacy and care as those of a theodolite. The upper bearing, 12 inches diameter, consists essentially of a large "plumber-block," in which slide, in horizontal grooves, two massive wedge-shaped prisms carrying gun-metal blocks, bearing the axis, and which are acted upon by screws, so that motion can be given to them in a horizontal direction. (See Fig. 1.) This arrangement is for finally adjusting the axis into parallelism with the pole of the earth. It will be readily seen that by these two screws any required motion can be given: thus forcing both screws in or out equally, will give a vertical motion up or down; FIG. 1. while by advancing one screw in proportion as the other is withdrawn, a horizontal motion results in the direction of the retreating screw. 50 The lower bearing rests in a block of gun-metal bored to fit the axis, and cut away for about 70° at the bottom to give it the property of a Y. The axis terminates in a flat-polished piece of chilled cast-iron, 5 inches diameter, bearing against a flat cushion of bell-metal, which cushion, of a spherical shape on its lower face, rests in a spherical cup. Therefore as the axis is adjusted into its proper direction by the screws in the upper bearing, the motion of the bell-metal cushion in its cup ensures a perfectly even bearing between it and the chilled iron and bell-metal surfaces. Now as the weight of the instrument as it rests on these bearings, of 12 and 6 inches diameter, amounts to about 8 tons, it follows that the friction, if not disposed of in some manner, would be so considerable as to render the instrument quite unmanageable; but in all these bearings there is only about th orth part of the weight really resting in the bearings themselves, whilst the remainder is supported by apparatus which reduces the friction to a minimum. In this manner are obtained great freedom of motion, less wear, and at the same time all the steadiness of the Y bearing; practically, in fact, more steadiness than if the whole weight were allowed to rest, for then it is found that an inclination exists to ride up on the forward side. Accordingly, close above the lower bearing is placed a sector working on a hardened steel pin, and forced up by a screw and strong laminæ of springs on which the axis rolls with a pressure of about 4 tons. Now as the radius of the sector is 27 inches, and the half diameter of the pin is inch, it follows that the friction is reduced in the proportion of, or about 62 to 1. The same principle is carried out in the upper bearing, but here the weight not being so excessive, a roller of 8 inches diameter was thought sufficient, acted upon by a lever and weights hanging on the west side of the pier. In addition to this, to take off some of the pressure on the toe-pieces -more to prevent danger of biting than for the purpose of reducing friction-the polar axis is laid hold of by a very peculiar steel link chain, so constructed that the links can twist a small quantity on each other without producing friction, which chain is held up by a trussed lever of the proportion of about 8 to 1, acted upon by weights at the back of the pier amounting to about half-a-ton, so as to relieve about 4 tons of end pressure. So effective is this arrangement, that a force of 5 pounds at a point 20 feet from the centre of motion, is sufficient to move this mass of 8 tons. W FIG. 2. R The arrangement for the relief of the friction in the declination axis is necessarily much less simple. It will readily be seen by an inspection of Fig. 2, which we may suppose to represent the end of the declination axis, and which for simplicity is drawn for a latitude of 45°, P A being the direction of the polar axis, that if we place the axis on Ys at the points say a and b, when we reverse the instrument to the other side of the meridian these bearing-points will be at respectively c and d, and the axis would roll out of its bearings; or if we put the Ys at mean points e and f, all the weight would be on one or other of the Ys, as the instrument is reversed from one side of the meridian to the other, and it would be quite impossible to relieve any portion of the friction by apparatus similar to that used for the polar axis, for what would be right on one side would be quite wrong on the other. Up to the present time this has been the great objection to this form of equatorial; for as explained above, the axis had necessarily to be put into the entire collared bearings without any possible relief of friction, the force necessary to move the telescope, if of any considerable size, became so great as to oblige the constructor to make the bearings of the declination axis very small, and consequently rendered the support of the telescope weak and unsteady. Only in this telescope, and in one other (also constructed by Mr. Grubb) which is now mounted at Dunsink Observatory, near Dublin, the object glass being presented by the late Sir James South to the University of Dublin, has this difficulty, one of the very few objections to this form of equatorial, been overcome. Referring to Fig. 2, suppose the weight W, acting perpendicularly, to be resolved into two forces, one, P, acting parallel and the other, R, at right angles to the polar axis. The first, P, inasmuch as it is parallel to, and along the axis, is constant in its direction to that axis as it turns round. The second, R, varies in its direction with respect to the axis as it turns, but is constant as regards any vertical line. Now if R could be disposed of, it is evident that the case would be that of an equatorial at the pole of the earth, all the pressure being in a direction parallel to the polar axis. This, then, could be readily relieved by anti-frictional apparatus. To understand how this R is disposed of, refer to Figs. 3, 4, and 5. Fig. 3 shows the apparatus, by which this is accomplished, taken asunder. It consists of two nearly semicircular cast-steel rings, A and B, very strong, between the jaws of which, when together, are held the gun-metal carriages, shown at E E', carrying three rollers each, one pair each, a a' (one only of each pair can be seen in the figure), and one single, e e', at right angles to the other two. This apparatus when bolted together embraces the declination axis where its axis crosses that of the polar axis (where the declination axis is formed as at Fig. 4). The two pairs of rollers a a' (Fig. 3) work on the rings A A' (Fig. 4), whilst the third rollers e e' (Fig. 3) work in the groove B (Fig. 4). The lower half-ring has a steel pin d (Fig. 3), which rests in a cavity prepared for it at the bottom of the polar axis, where it forms the fulcrum of the lever, while the upper half-ring is prolonged into a stiff trussed bar which runs up through the polar axis and projects at the upper end, where it is acted upon by a sector and lever, which by the application of sufficient weights, which hang down the eastern side |