ON THE MEASDREMENT OV THE LUNAU DISTURBANCE OF GRAVITY. 95 



of compulsory intermissions, we have continued to work at this subject. 

 My brother Horace and I have always discussed together the plan on 

 which to pi'oceed ; but up to the present time much the larger part of 

 the work has consisted in devising mechanical expedients for overcoming 

 difficulties. In this work he has borne by very far the larger share ; and 

 the apparatus has been throughout constructed from his designs, and 

 under his superintendence, by the Cambridge Scientific Instrument 

 Company. 



Near the corner of a stone-paved ground-floor room in the Cavendish 

 Laboratory there stands a very solid stone gallows, similar to, but rather 

 more massive than, the one at Glasgow. As it did not appear thoroughly 

 free from rigid connection with the floor, we had the pavement raised all 

 round the piers, and the earth was excavated from round the brick 

 basement to the depth of about 2 feet 6 inches, until we were assured 

 that there was no connection with the floor or walls of the room, excepting 

 through the earth. The ditch, which was left round the piers, was 

 found very useful for enabling us to carry out the somewhat delicate 

 manipulations involved in hanging the mirror by its two silk fibres. 



Into the middle of the flat ends of one of our copper weights (which 

 weighed 4,797 grammes, with spec. gr. 8-91) were screwed a pair of 

 copper plugs ; one plug was square-headed and the other pointed. Into 

 the centre of the square plug was soldered a thin copper wire, just capable 

 of sustaining the weight, and intended to hang the pendulum. 



A stout cast-iron tripod was made for the support of the pendulum. 

 Through a hole in the centre of it there ran rather loosely a stout iron rod 

 with a screw cut on it. A nut ran on the screw and prevented the rod 

 from slipping through the hole. The other end of the copper wire was 

 fixed into the end of the rod. 



The tripod was placed with its three legs resting near the margin of 

 the circular hole in the centre of the lintel of the gallows. The iron rod 

 was in the centre of the hole, and its lower end appeared about six inches 

 below the lower face of the lintel. The pendulum hung from the rod by 

 a wire of such length as to bring the spiked plug within a few inches of 

 the base of the gallows. This would of course be a very bad way of 

 hanging a pendulum which is intended to swing, but in our case the dis- 

 placements of the end of the pendulum were only likely to be of a magni- 

 tude to be estimated in thousandths or even millionths of an inch, and it 

 is certain that for such small displacements the nut from which the 

 pendulum hung could not possibly rock on its bearings. However, in 

 subsequent experiments we improved the arrangement by giving the nut 

 a flange, from which there projected three small equidistant knobs, on 

 which the nut rested. 



The length of the pendulum from the upper juncture with the iron rod 

 down to the tip of the spike in the bob was 148-2 cm. 



An iron box was cast with three short legs, two in front and one 

 behind ; its interior dimensions were 15 x 15 x 17i cm. ; it had a tap at 

 the back; the front face (15 X 17h) was left open, with ai-rangements for 

 fixing a plate-glass face thereon. "The top face (15 x 17i) was pierced 

 by a large round hole. On to this hole was cemented an ordinary 

 earthenware 4-inch drain pipe, and on to the top of this first pipe there 

 was cemented a second. The box was thus provided with a chimney 

 144 cm. high. The cubic contents of the box and chimney were about 

 3^ gallons. 



