1894.] On the Newtonian Constant of Gravitation. 131 



June 7, 1894. 

 The LORD KELVIN", D.C.L., LL.D., President, in the Chair. 



A List of the Presents received was laid on the table, and thanks 

 ordered for them. 



The following Papers were read : 



I. " On the Newtonian Constant of Gravitation." By C. V. 

 BOYS, F.R.S., A.R.S.M., Assistant Professor of Physics, 

 Royal College of Science, South Kensington. Received 

 May 31, 1894. 



(Abstract.) 



The Newtonian constant of gravitation G, i.e., the force in dynes 

 between 2 grams of matter 1 cm. apart, has been determined with a 

 very accurately constructed piece of apparatus, designed on the lines 

 which I laid down in my paper on the Cavendish experiment (' Roy. 

 Soc. Proc.', vol. 46, p. 293). The important dimensions are approxi- 

 mately 



Distance between centres of lead balls in plan . . 6 in. 



gold . . 0-9 in. 



Diameter of lead balls 4^ in. 



gold 0-2 and O25 in. 



Difference of level between right and left sides . 6 in. 



The lead balls were hung by phosphor bronze wires from pillars in 

 the lid of the apparatus, and the gold balls by quartz fibres from the 

 ends of the " beam mirror." The beam mirror was supported by a 

 quartz fibre, 17 in. from a torsion head. An elaborate system of 

 screens protected the apparatus from temperature variations. 



An " optical compass " of extreme precision was employed in 

 measuring the horizontal distances between the fibres and between 

 the wires, which alone among the geometrical magnitudes need be 

 known with a very high degree of precision. 



The scale was 9 ft. long, divided into 50ths of an inch. It was 

 placed at a distance equal to 14,000 divisions. It could be read with 

 certainty to 1/10 division. The deflections varied according to the 

 circumstances of each experiment from 351 to 577 divisions, and the 

 squares of the periods from 35,431 to 58,519 sees. 2 



K 2 



