﻿180 DESCRIPTION OF THE OBSERVATORY AT CAMBRIDGE. 



A circle of granite surmounts the walls. On this is secured an iron plate ten inches 

 in width, hollowed in the middle ; a similar track is laid on and bolted to the under sec- 

 tion of the dome ; and between them are placed eight iron spheres, eight inches in 

 diameter, upon which the dome rolls. 



The openings for the telescope are nearly five feet in width, extending from three 

 degrees beyond the zenith to as many below the horizon, the entire aperture being 

 available. The shutters are divided into three compartments, the two upper moving 

 on an iron railway, with rollers at distances of a foot apart, to relieve the friction. The 

 shutters are worked by means of endless chains, acting in toothed wheels, turned by a 

 crank and pinion ; the leading of these chains, as well as the position of the crank, is 

 shown in Plate V., Fig. 1. The third and lowest division of the shutters is subdivided 

 into three parts, each of which is counterpoised by weights traversing within the dome, 

 and acting in the same manner as those of ordinary window-sashes. The frames of 

 all are of ash, covered with copper sheathing, and they are perfectly weather-proof. The 

 machinery occupies but little room, and the whole is easily managed. 



Plate V., Fig. 1, is a section of the dome, intended to illustrate the arrangement for 

 working the shutters. At a, b, c, d, they are represented as closed ; a and b are simi- 

 lar, each being nine feet long. The frames are of two-inch-square ash; there are in each 

 nine cross-bars, besides the end-pieces, and the whole is covered simply by copper, 

 painted, in order that they may be as light as possible consistently with the requisite 

 strength. 



A transverse section of a is shown in Fig. 2, where, at a a, &c, the shutter bears, 

 by iron rollers inserted into the framework of shutters, upon the dome ; and at b' b', on 

 each side, near the ends, are like rollers, to relieve the friction which might take place 

 during high winds, causing a lateral pressure. 



In Fig. 1, h h, &c. shows the track of the chains by which the shutters are raised 

 and lowered. Atf there is an iron arm, which is secured to the upper shutter, and pro- 

 jects downwards within the middle one. To this arm one end of the chain is secured, 

 and leads forward from f, passes round a pulley at g, and then back over pulleys at h h, 

 &.C., keeping nearly the curvature of the dome, and downward to t, where it is receiv- 

 ed on a toothed pulley of seven inches diameter, like that shown in Plate IV., Fig. 2, let- 

 ter i. The chain then leads upward, supported in place by rollers, in the direction 

 h h, until it reaches f, where it is riveted to the other end, thus forming a continuous 

 chain. It will be borne in mind, that there is precisely a like arrangement of chains and 

 pulleys for each side of each shutter, and by reference to the plate it will be understood 

 that, on turning the crank at i forward, the shutter will be opened ; if turned backward, 

 the shutter will be closed. 



