. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. 208 RECENT STUDIES IN GRAVITATION. I The they used was a nioditied kiud of \s apparatus (tig. 8). Two small gold masses, in the form of short vertical wires, each 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 posit
. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. 208 RECENT STUDIES IN GRAVITATION. I The they used was a nioditied kiud of \s apparatus (tig. 8). Two small gold masses, in the form of short vertical wires, each 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 appar
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