206 RECENT STUDIES IN GRAVITATION. 



of moving the attracting mass, the attracted mass was moved. Masses 

 of 1 kilogram each were put first, say, one in the upper right-hand pan, 

 the other in the lower left-hand pan, when the pull of the lead block 

 made the right hand heavier and the left hand lighter. Then the 

 weights were changed to the lower right hand and the upper left hand, 

 when the pulls of the lead pile were reversed. When Ave remember 

 that in my experiment a lowering of the hanging sphere by 1^ inches 

 would give an effect as great as the pull I was measuring, it is evident 

 that here the approach to and removal from the earth by over 2 meters 

 woidd produce very considerable changes in weight, and, indeed, 

 these changes masked the effect of the attraction of the lead. Prelimi- 

 nary experiments had, therefore, to be made before the lead pile was 

 built up, to find the change in weight due to removal from upper to 

 lower pan, and this change had to be allowed for. The quadruple 

 attraction of the lead pile came out at 1.3664 mgm., and the mean 

 densit}^ of the earth at 5.505. 



This agrees nearly with m^^ own result of 5.49, and it is a curious 

 coincidence that the two most recent balance experiments agj-ee very 



Fig. 5.— Paramagnetic sphere placed in a Fig. 6.— DiainaRnetic sphere placed in 



previously straight field. a previously straight field. 



nearly at, say, 5.5, and the two most recent Cavendish experiments 

 agree at, say, 5.53, ])ut 1 confess I think it is merely a coincidence. I 

 have no doubt that the torsion experiment is the more exact, though 

 probably an experiment on different lines was worth making, and I 

 am quite content to accept the value 5.527 as the standard value for the 

 present. 



And so the latest research has amply verified Newton's celebrated 

 guess that "the (Quantity of the whole matter of the earth may be five 

 or six times greater than if it consisted all of water." 



I now turn to another line of gravitational research. AVhen we 

 compare gravitation with other known forces (and those which have 

 been most closely .studied are electric and magnetic forces) we are at 

 once led to in(}uire whether the lines of gravitative force are always 

 straight lines radiating from or to the mass round which they center, 

 or whether, like electric and magnetic lines of force, they have a pref- 

 erence for .some media and a distaste for others. We know, for 

 example, that if a magnetic sphere of iron or colialt or manganese is 

 placed in a previously straight field its permeability is greater than 



