204 



RECENT STUDIES IX GRAVITATION. 



Cavendish experiment turned from a horizontal into a vertical plane, 

 and in which the torsion balance is replaced by the common balance. 

 This method occurred about the same time to the late Prof. IT. 

 Jolly and myself. The principle of my own experiment" will be 

 sufficiently indicated by fig. 3. A big bullion balance with a 4-foot 

 beam had two lead spheres. A B, each about 50 pounds in weight, hang- 

 ing from the two ends in place of the usual scale pans. A large lead 

 sphere, M. 1 foot in diameter and weighing about 350 pounds, was 

 brought first under one hanging weight, then under the other. The 

 pull of the lead sphere acted first on one side alone and then on the 



Common balance experiment (Poynting). 



other so that the tilt of the balance beam when the sphere was moved 

 round was due to twice the pull. By means of riders the tilt and there- 

 fore the pull was measured directly as so much increase in weight. 

 This increase, when the sphere was brought directly under the hanging 

 weight with 1 foot between the centers, was about one-fifth mgm. in a 

 total weight of 20 kilograms, or about I in 100,000,000. If. then, a 

 sphere one foot away pulls with 1 10 8 of the earth's pull, the earth 

 being on the average 20,000,000 feet away, it is easy to see that the 

 earth's mass is calculable in terms of the mass of the sphere, and its 



a Phil. Trans. 182, 1891, A, p. 565. 



