188 RECORD OF SCIENCE FOR 1887 AND 1888. 



ASTRONOMICAL INSTRUMENTS. 



The 36'inch LicJc telescope. — The great equatorial was mounted in 1887. 

 The visual objective has a clear aperture of 36 inches and focal length 

 of 50 feet 6 inches. The flint disk for this objective was obtained bj^ 

 Clarlc from Fell & Co., of Paris, in 188:}, the crown in 1885, and the ob- 

 jective was completed and delivered by Clark in 188G. These two disks 

 are separated in their cell by a space of G.5 inches. In 1887 a third 

 (crown) lens of 33 inches aperture and 40 feet focus was procured as a 

 "photographic corrector." The mounting was made by Warner & 

 Swasey, of Cleveland, Ohio, and put in place in the autumn of 1887. 

 The pier is a hollow cast-iron column of rectangular section, and for 

 convenience in transit is built up of five sections, bolted together by 

 internal flanges. At the top the column measures 4 feet by 8 feet, in- 

 creasing to 5 feet by 9 feet at the floor line, and then spreading out rapidly 

 to a base 10 feet by 16 feet bearing on a masonry foundation. At this 

 point the weight is distributed over a series of large steel screws, which 

 aff'ord the means of adjusting the position of the polar axis. At the top 

 of the column is the liead carrying the bearings for the polar axis, and 

 around the head is a balcony reached by a spiral staircase. The inter- 

 section of polar and declination axes is 37 feet above the base. Column 

 and head weigh together 21 tons, and the total weight of the telescope 

 is 40 tons. The tube consists of a central section of cast-iron, strongly 

 ribbed, to which two sections of sheet-steel are attached. The diameter 

 of the tube is 4 feet at the center, 38 inches at the object-glass end, and 

 36 inches at the eye end, while the sheet-steel portions vary from one- 

 eighth of an inch thick at the center to one-twelfth of an inch thick at 

 the ends. When the telescope is in a horizontal position the flexure of 

 the tube with the object glass and permanent counterbalance is one- 

 eighth of an inch only, while with a load of 1 ton added at each end the 

 flexure is increased to one-quarter of an inch. 



The polar axis is of steel, 10 feet long, 12 inches in diameter at its 

 upper and 10 inches at its lower end, with a Cinch hole through its 

 entire length; the bearings are of Babbitt luetal, relieved by anti-friction 

 rolls. The declination axis is also of steel, 10 feet inches long, 10 

 inches tapering to 9| inches diameter, with a 4-inch hole running 

 through it; Babbitt-metal bearings, with anti friction rolls, upon hard 

 steel balls are used, as in the polar axis. To facilitate counteri)oising 

 the telescope is arranged for carrying always its maximum load, so that 

 when the ijhotographic corrector, the spectroscope, or any physical ap- 

 paratus is attached an equivalent weight is taken oft" at the same place. 

 There are three regular tinders of 6, 4, and 3 inches ajierture; in addi- 

 tion to these the 12inch equatorial can be quickly attached as a pointer 

 for photographic work if necessary. The hour circle on the polar axis 

 is 3 feet in diameter, and has coarse graduations to 5 minutes, and large 

 figures on its outer edge, while on the northern fiice it is graduated to 

 20 seconds. Attached to the head, at the upper end, is a fixed hour 



