202 



RECENT STUDIES IN GKAVITATION. 



distiinco between t.hoin aiul the force (I M, Mj?'^, of course Cavendish's 

 result gives (x, or, knowing- the attraction of a ))io- sphere on a ball, 

 and knowing- th<^ attraction of the earth on the same ball — that is, its 

 weight — the experiment gives the mass of earth in terms of that of 

 the big sphere, and so its mean density. This experiment has often 

 been repeated, but I do not think it is too much to say that no advance 

 was made in exactness till we come to quite recent work. 



B}^ far the most remarkal>le recent study in gravitation is Professor 

 Boys's beautiful form of the Cavendish experiment, a research which 



Fig. 2. — Boys's apparatus. 



stands out as a model in l)eauty of design and in -exactness of execu- 

 tion (tig. 2). But as Professor Boys has described his experiment 

 already in this theater,'' it is not necessary for me to more than refer 

 to it. It is enough to say that he made the great discovery, obvious, 

 perhaps, %Yhen made, that the sensitiveness of the apparatus is increased 

 by reducing- its dimensions. He therefore decreased the scale as far 

 as was consistent with exact measurement of the parts of the appa- 

 ratus, using a torsion rod, itself a mirror, only 2 inches long, gold 

 balls, m i/i, only i inch in diameter, and attracting lead masses. M M, 



»Proc. Eoyal Institution, XIV, part 2, 1894, p. 353. 



