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 5,000l. 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. 6 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, 1 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. 30 00 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 or th 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 |