RECENT STUDIES IN GRAVITATION. 



207 



the air it replaces, and the lines of t'oree crowd into it, as in rig. 5. 

 The magnetic action is then stronger in the presence of the sphere near 

 the ends of a diameter parallel to the original course of the lines of 

 force and the lines are deflected. If the sphere be diamagnetic, of 

 water, or copper, or ])ismutli. the permeability being less than that 

 of air, there is an opposite eliect, as in rig. »>. and the rield is weakened 

 at the end of a diameter parallel to the lines of force, and again the 

 lines are detiected. Similarly a dielectric bod}' placed in an electric 

 rield gathers in the lines of force and makes the held where the lines 

 enter and leave stronger than it was before. 



If we inclose a magnet in a hollow box of soft iron placed in a mag- 

 netic rield, the lines of force are gathered into the iron and largely 

 cleared away from the in- 

 side cavity, so that the mag- 

 net is screened from exter- 

 nal action. 



Now, conunon experience 

 might lead vis at once to say 

 that there is no very con- 

 siderable etfect of this kind 

 with gravitation. The evi- 

 dence of ordinary weighings 

 may perhaps be rejected, 

 inasmuch as both sides will be equally affected as the balance is 

 commonly used. But a spring balance should show if there is any 

 large etfect when used in different positions above different media or 

 in different inclosures, and the ordinary balance is used in certain 

 experiments in which one weight is suspended beneath the balance 

 case and surrounded perhaps by a metal case or perhaps b}- a water 

 bath. Yet no appreciable variation of weight on that account has }^et 

 been noted, nor does the dii-ection of the vertical change rapidly from 

 place to place, as it would with A^arying permeability of the ground 

 below. But perhaps the agreement of pendulum results, whatever the 

 block on which the pendulum is placed and whatever the case in which 

 it is contained, gives the best evidence that there is no great gathering 

 in or opening out of the lines of the earth's force by different media. 



Still, a direct experiment on the attraction between two masses with 

 different media interposed was well worth\- of trial, and such an exper- 

 iment has lately been carried out in America by Messrs. Austin and 

 Thwing." The effect to l)e looked for will be understood from rig. 7. 

 If a mediiuii more permeable to gravitati(^i is interposed between two 

 bodies, the lines of force will move into it from each side, and the 

 gravitative pull on a body near the interposed medium on the side 

 away from the attracting body will be increased. 



-Effect of interposition of more permtjable meMlium 

 in radiating field of force. 



^Physical Eeview, V, 1897, p. 294. 



