﻿DESCRIPTION OF THE OHSERVATORY AT CAMBRIDGE. 187 



with spiral springs, to relieve the pressure of the pivots upon the Ys, have been applied 

 instead of the single wheel ; abutting pieces, with double thumb-screws, are placed al 

 the ends of the pivots on the bearing-plates, to secure the axis against lateral motion. 

 The frame of the instrument appears simple, firm, and symmetrical. The circles are 

 four feet in diameter, being cast in one piece, and are both graduated on silver, from 0° 

 to 360°, into five-minute spaces. 



The telescope has an object-glass, by Merz, of four and one eighth inches aperture, 

 and sixty inches focal length. The eight reading microscopes serve to bisect diametri- 

 cally both circles ; the five-minute spaces of the limbs are subdivided by the microme- 

 ters, a single division of the micrometer-head being equal to one second of arc, and may 

 be read by estimation to two tenths of a second. 



The length of the axis, between the shoulders of the pivots, is two feet two inches ; 

 the pivots are of steel, two and a half inches in diameter, and the same in length ; the 

 bearing-plates of the Ys are eight inches broad and twelve inches long ; they are secured 

 to the granite piers by screws passing into nuts, which are firmly imbedded with plaster 

 of Paris in the stone. 



The friction-wheels, for relieving the pressure of the axis-pivots upon the Y's, are sup- 

 ported by plates, eleven inches and a half by twelve inches, secured to the piers. These 

 plates carry also the screw of slow motion, which is brought into action by means of a 

 clamp embracing the axis of the telescope. Plate VI. is a perspective view of the me- 

 ridian-circle, mounted on its piers. The room in which it is placed is seventeen by 

 eighteen feet, and twelve feet high. A description of the shutters of the roof in this 

 room will answer for those of the prime-vertical, as they differ only in position. 



Plate V., Fig. 4, is a sectional view of the southern shutter. It extends down about 

 two feet on the north side, and is quite close at the apex, r. The movement is horizon- 

 tal, east and west, on rollers running on iron rails laid on the outside and inside of the 

 roof of the building, the bearings being at rrr and t. At s is a strong iron arm, secured 

 to the shutter ; the form and action of this iron arm are better seen in fig. 6 ; it is made 

 fast to the shutter at w. At the end, v, a rope is fastened and carried first forward toward 

 the opening, passing round the pulley x. It then leads back, over the pulley y, down to 

 z, where it is secured to the wheel z. The diameter of this wheel is proportioned to 

 the distance which the shutter requires to be moved, so that a turn and a half of the 

 crank, z' suffice to open or close the shutter. The other part of the rope, from v, leads 

 over the pulley y, and is wound round and fastened to the wheel z, in the opposite direc- 

 tion ; so that the turning of the crank z' in one direction serves to open, and in the other 

 to close, the shutter. 



