RECENT STUDIES IN GRAVITATION. 203 



only -li iiK^hos in diameter. The forci^ (<> l)o mciisui'od was loss than 

 1/5" X 10" orain. 



The exactness of liis work was increased l)y usiny as suspending 

 wire one of his quartz threads. It would l)e difficult to overestimate 

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

 discovery of the remarkable properties of these threads. 



One of the chief difficulties in the measurement of these small grav- 

 itational 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 diminution 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 disturlung air currents by suspend- 

 ing his torsion rod in a receiver which was nearly exhausted, the pres- 

 sure being reduced to about one two-hundredth of an atmosphere. 

 The gales which ha\'(^ been the despair of other workers were thus 

 reduced to such gentle breezes that theii- etfect was hardly noticeable. 

 His ai)pai"atus was nearh' a mean proportional ))etween that of Caven- 

 dish and BoA^s, his torsion rod being about 9 inches long, the balls 

 weighing 54 grams — less than 2 ounces — and the attracting masses 

 either 5 or 9 kilograms. 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 facili- 

 ties, far from workshops, and he had to make much of his appa- 

 ratus 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 fiber supplied to him by Professor Boys in place 

 of the somewhat untrustworth}' metal wire which he used in the work 

 alread}^ 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, 

 seeking the^^Tilue of (t, ijiformation which would be as useful on ^lars 

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

 perhaps we ma}" 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 densit}^ 

 5.527 times the density of water, agreeing therfeore to 1 in 5,000. 



There is another mode of proceeding which ma}"^ be regarded as the 



* DenkHchriften der ]\Iatli. Wiss. Classe der Kais. Akad. der Wissenschaften Wiea, 

 LXIV, 1896. 



