﻿DESCRIPTION OF THE OBSERVATORY AT CAMERIDGE. 179 



dations of the outer walls, and at b b those of the inner walls ; the latter support the 

 dome. The foundation of the great pier is a grouting, composed of cement and 

 coarse gravel, twenty-two feet in diameter at its base, which is twenty-six feet below 

 the natural surface of the ground ; the depth of this mass is ten feet. When the grout- 

 ing had become well consolidated, the pier was commenced with blocks of granite four 

 feet in length by two in thickness. 



The pier rises thirty-three feet, to the floor of the dome, in the form of a frustum 

 of a cone, twenty feet in diameter at the base and ten feet at the top. It is surmounted 

 by a capstone of ten feet in diameter and twenty-two inches in thickness. Upon this 

 stands the granite tripod support, of eleven tons' weight ; its height is eleven feet. 

 From the top of the tripod to the ceiling of the dome is sixteen, feet ; the ball on the 

 outside is one foot eight inches above the ceiling ; making the entire height from the 

 foundation seventy-three feet. In building the pier, the mason fitted each block to a 

 steady bearing before any cement was inserted. 



The form of the granite pedestal, to which the bed-plate of the equatorial mounting 

 of the telescope is attached, will be best understood by reference to Plate II. The base 

 of this pedestal is seven feet one inch on the east and west sides, four feet five inches 

 on the north, and three feet five inches on the south sides. It rests on the cap-stone 

 by three bearings, so situated in regard to the centre of gravity of the whole block and 

 telescope that each bearing supports a nearly equal share of the weight. These bear- 

 ings are protuberances left when the stone was worked. This method I have adopt- 

 ed with all the piers, as it insures steadiness and admits of easy change of position. 

 Of this we have had an instance in the mounting of the great equatorial, where the 

 azimuth screws came home before the adjustments were completed ; by the application 

 of a jack-screw to the base in the line of two of the supports, the whole pedestal and 

 telescope were moved into the required position round the third support as a centre, 

 without perceptible jar or risk of injury. 



The piers for the prime-vertical transit, Plate I., Fig. 2, and for the transit-circle, 

 have their foundations of grouting at eighteen feet below the surface. In these, the 

 grouting is inclosed in a pyramidal casing of brick-work, curving inward, and support- 

 ing a granite block eight feet long, four feet wide, and twenty-two inches thick ; on 

 this block rest the tripod-pillars supporting the Vs of the instruments. 



The dome covering the great equatorial is a hemisphere of thirty feet interior diam- 

 eter, framed with stout ribs of plank, and covered with copper on the outside, on a 

 sheathing of thin boards. The ceiling within is of wood, the air being allowed to circu- 

 late between the inner and outer coverings. We have been gratified to find that the 

 internal temperature quickly approximates to the external, during sudden changes. 



