282 Professor John H. Poynting [Feb. 23, 



decreased the scale as far as was consistent with exact measurement 

 of the parts of the apparatus, using a torsion rod, itself a mirror, 

 only 2" long, gold balls, m m, only J" in diameter, and attracting lead 

 masses, M M, only 4^" in diameter. The force to be measured was less 

 than 1/5 x 10 6 grain. 



The exactness of his work was increased by using as suspending 

 wire one of his quartz threads. It would be difficult to overestimate 

 the service he has rendered in the measurement of small forces by 

 the discovery of tbe remarkable properties of tbese threads. 



One of the chief difficulties in the measurement of these small 

 gravitational pulls is the disturbances which are brought about by 

 the air currents, which blow to and fro and up and down inside the 

 apparatus, producing irregular motions in the torsion rod. These, 

 though much reduced, are not reduced in proportion to the diminu- 

 tion of the apparatus. 



A very interesting repetition of the Cavendish experiment has 

 lately been concluded by Dr. Braun * at Mariaschein in Bohemia, in 

 which he has sought to get rid of these disturbing air currents by 

 suspending his torsion rod in a receiver which was nearly exhausted, 

 the pressure being reduced to about ^iro °f an atmosphere. The 

 gales which have been the despair of other workers were thus 

 reduced to such gentle breezes that their effect was hardly noticeable. 

 His apparatus was nearly a mean proportional between that of 

 Cavendish and Boys, his torsion rod being about 9" long, the balls 

 weighing 54 gms. — less than two ounces — and the attracting masses 

 either 5 or 9 kgins. His work bears internal evidence of great care 

 and accuracy, and he obtained almost exactly the same result as 

 Professor Boys. 



Dr. Braun carried on his work far from the usual laboratory 

 facilities, far from workshops, and he had to make much of his 

 apparatus himself. His patience and persistence command our 

 highest admiration. 



I am glad to say that he is now repeating the experiment, using 

 as suspension a quartz fibre supplied to him by Professor Boys in 

 place of the somewhat untrustworthy metal wire which he used in 

 the work already published. 



Professor Boys has almost indignantly disclaimed that he was 

 engaged on any such purely local experiment as the determination of 

 the mean density of the earth. He was working for the Universe, seek- 

 ing the value of G, information which would be as useful on Mars or 

 Jupiter or out in the stellar system as here on the earth. But 

 perhaps we may this evening consent to be more parochial in our 

 ideas, and express the results in terms of the mean density of the 

 earth. In such terms then both Boys and Braun find that density 

 5*527 times the density of water, agreeing therefore to 1 in 5000. 



* Denkschriften der Math. Wiss. Classe der Kais. Akad. der Wissenschaften 

 Wieu, lxiv. 1896. 



