﻿DESCRIPTION OF THE OBSERVATORY AT CAMBRIDGE. 181 



At k k, Plate V., Fig. 1, are two iron rails secured to the outside of the dome ; they 

 receive the shutter a when it is rolled back ; there are separate rollers for this track, seen 

 at c' c', &c, Fig. 3. They have proved safe guides, when the dome has been much 

 encumbered with ice and snow. Fig. 3, at a' a', &c, are iron rollers about one inch in 

 diameter ; they bear the weight of the shutter, resting on the frame of the dome at the 

 sides of the opening. 



The arrangements for the middle shutter are similar to those for the upper one ; the 

 only difference consists in placing the iron arm to which the chain is fastened at the 

 upper, instead of the lower, part, and leading its chains down to the pulley at o, Fig. 1. 

 The chain is then carried up along the edge of the opening, round the ceiling of the 

 dome, and down to i, where there is another toothed pulley to receive it. 



Plate V., Fig. 1, at n is given an edge view of the iron frame supporting the wheels 

 and pinions for moving the chains of the shutters. This frame is six feet two inches wide 

 by twelve feet high, and is firmly bolted at its upper part to the dome, at p. The upright 

 part is of bar-iron three inches wide by three quarters of an inch thick ; the cross-bracing 

 bars are two inches wide by half an inch thick. At q are bolt-holes, intended for attach- 

 ing the observing-chair to the frame to be carried round with the dome. It is not, how- 

 ever, used, as the driving-wheel of the chair is found to be more convenient, and brings 

 this motion immediately under the control of the observer. 



The axles of the wheels at i are six feet long, and near the ends have toothed pul- 

 leys, like the one shown in Plate IV., Fig. 2, letter t. There are four of these pullevs, 

 each of them seven inches in diameter. The axle carrying the chain of the upper shut- 

 ter is a hollow cylinder, and the axle of the lower shutter passes through it. The power- 

 gearing of both is on the right-hand side of the frame, and consists of two iron wheels 

 (one for each axle) of eleven inches in diameter, and a double-headed pinion of three 

 inches in diameter. To the pinion-arbor is attached a crank of one foot purchase. The 

 pinions have an end motion, that they may readily be changed from one wheel to the 

 other, the same crank moving either shutter up or down. 



For turning the dome, a series of toothed iron plates, with a curvature of fourteen 

 feet nine inches radius, is fastened to its lower section, forming a contrate wheel of 

 twenty-nine feet and a half in diameter. Into this is geared a wheel four feet in diam- 

 eter, acting vertically in the recess at the southeast corner. The weight of the dome 

 is about fourteen tons, and it can be moved through an entire revolution by a single 

 person in thirty or forty seconds. 



In addition to the security resulting from the weight of the dome constantly tending 

 to bring the balls to the middle of the curved rail, and consequently to produce a coinci- 



