280 



Professor John H. Poynting 



[Feb. 23, 



distances gave the mountain deflection. It is not surprising that, 

 working in snowstorms at one station, and in sandstorms at the other, 

 Bouguer obtained a very incorrect result. But at least he showed 

 the possibility of such work, and since his time many experiments 

 have been carried out on his lines under more favourable conditions. 

 Now, however, I think it is generally recognised that the difficulty 

 of estimating the mass of a mountain from mere surface chips is 

 insurmountable, and it is admitted that the experiment should be 

 turned the other way about and regarded as an attempt to measure 

 the mass of the mountains from the density of the earth known by 

 other experiments. 



These other experiments are on the line indicated by Newton in 

 his calculations of the attraction of two spheres. The first was 

 carried out by Cavendish. 



Fig. 1. — Cavendish's Apparatus. 



In the apparatus (Fig. 1) he used two lead balls, B B, each 2" in 

 diameter. These were hung at the end of a horizontal rod 6' long, 

 the torsion rod, and this was hung up by a long wire from its middle 

 point. Two large attracting spheres of lead, W W, each 12" in 

 diameter, were brought close to the balls on opposite sides so that 

 their attractions on the balls conspired to twist the torsion rod round 

 the same way, and the angle of twist was measured. The force could 

 be reckoned in terms of this angle by setting the rod vibrating to and 

 fro and finding the time of vibration, and the force came out to less 

 than 1/3000 of a grain. Knowing M x M 2 and r the distance between 

 them and the force G M 1 M 2 /r 2 , of course Cavendish's result gives G, 

 or knowing the attraction of a big sphere on a ball, and knowing 

 the attraction of the earth on the same ball, that is its weight, the 



