208 



RECENT STUDIES IN GRAVITATION. 



I 



The apparatu.s they used was a nioditied kiud of Boy.s\s apparatus 

 (tig. 8). Two small gold masses, in the form of short vertical wires, 

 each 0.4 gm. in weight, were an-anged at different levels at the ends 

 virtually of a torsion rod s mm. long. The attracting masses Mj M^ 

 were lead, each about 1 kgm. These were first in the positions shown 

 by black lines in the figure, and were then moved into the positions 

 shown by dotted lines. The attraction was measured tirst when merely 

 the air and the case of the instrument intervened, and then when vari- 

 ous slabs, each 8 cm. thick, 10 cm, wide, and 29 cm. high, were inter- 

 posed. With screens of lead, zinc, mercury, water, alcohol, or 

 glj^cerin, the change in attraction was at the most about 1 in 500, 



and this did not exceed 

 the errors of experiment. 

 That is, they found no evi- 

 dence of a change in pull 

 with change of medium. If 

 such change exists, it is not 

 of the order of the change 

 of electric pull with change 

 of medium, l)ut something 

 far smaller. Perhaps it 

 still remains just possible 

 that there are variatiojis of 

 gravitational permeability 

 com])aral)le with the varia- 

 tions of magnetic perme- 

 ability ill media such as 

 water and alcohol. 



Yet another kind of effect 

 might be suspected. In 

 most crystalline substances 

 the physical properties are 

 different along different di- 

 rections in a crystal. They expand differently, they conduct heat diff'er- 

 entl}^ and the}^ transmit light at different speeds in different directions. 

 We might, then, imagine that the lines of gravitative force spread out 

 from, say, a crystal sphere unequally in different directions. Some 

 years ago, Dr. Mackenzie" made an experiment in America, in which 

 he sought for direct evidence of such unequal distribution of the lines 

 of force. He used a form of apparatus like that of Professor Boys 

 (tig. 2), the attracting masses being calc spar spheres about 2 inches in 

 diameter. The attracted masses in one experiment were small lead 

 spheres about one-half gm. each, and he measured the attraction 

 betweeu-the crystals and the lead Avben the axes of the crystals were 



Fk;. s. — Experiment on gravitative permeabilitN 

 and Tliwing ). 



*^ Physical Review, II, 1895, p. 321. 



