﻿182 DESCRIPTION OF THE OBSERVATORY AT CAMBRIDGE. 



dence of the centres of the upper and lower circles, eight iron braces are fastened to the 

 wall, seen in Plate II., and present rollers of three inches in diameter to the interior of 

 the dome. These are designed to prevent an accumulation of oscillation during high 

 winds, rather than to act as guides on ordinary occasions. The form of these and of a 

 part of the machinery is exhibited in Plate II. 



Plates III. and IV. represent perspective and sectional views of the observing-chair. 



Plate IV., Fig. 1, at a a is seen a portion of the wooden frame ; b b is a section of the 

 box confining the counterpoise weight c, attached to the chain d d, which passes over 

 a pulley at f, and is secured to the chair at e. There is the same arrangement on the 

 opposite side. The two balance-weights exceed the weight of the chair by one hun- 

 dred pounds. The chain g g is attached to the upper extremity of the iron rail at h. 

 It passes round the toothed wheel i, and then upwards over the plain iron pulley at k, 

 thence downwards till it is made fast to the arm of the axle of the tension weight, the 

 purpose of which is to preserve constantly a safe gearing of the chain in the toothed 

 wheel i. The action of the chain and pulleys is seen in Fig. 2. 



The chains, side and edge views of which are given in Fig. 3, are made after the 

 same manner as the fusee-chain of a watch ; the links are each an inch and a half 

 long, and, being all struck out in the same die, are uniform in size. The teeth in the 

 wheel i have spaces corresponding to a link of the chain, and are cut down in the middle, 

 as seen in Figs. 2 and 4, to admit the inner connecting link, so as to allow the outside 

 links of the chain to fall into the spaces between the teeth of the wheel, giving a per- 

 fectly safe holdfast for the chain. 



In Fig. 4 the wheel for elevating the chair is seen attached to the shaft m ; this 

 shaft is turned by means of the endless screw n, Fig. 5, working in the wheel o, as seen 

 in Figs. 4 and 5. On the top of the shaft p, Fig. 5, is a round table, q, serving as a 

 convenient stand for the observer's note-book, &:c. 



It will be seen that by turning the handle r, motion will be communicated to the 

 shaft m, which is attached to the seat of the chair in such a manner as to elevate or 

 lower the chair as occasion may require. The endless screw n was selected for giving 

 the motion, in preference to any form of ratchet or lever work, for its security against the 

 accidents to which the latter are always liable. 



In order to make the eye of the observer move in a circle concentric with that de 

 scribed by the eye-piece of the telescope, and at the same time to preserve the horizon- 

 tal position of the seat, two curved iron rails are secured to the frame on each side of 

 the chair. On these rails the chair moves, being supported in front and back on rollers, 

 at s s and t t. To fulfil the required conditions, the radius of curvature of each rail was 



