9()8 KINETIC THEORIES OF GRAVITATION. 



formed is proportionate to the pressure of the spring multiplied by the 

 space through which the pressure can act. The potential element, 

 which in this case decreases as work is performed, is the tension or 

 elastic force of the spring. Space is necessary simply to allow this ten- 

 sion to expend itself in work. It is this tension, not space, which re- 

 appears as work or kiuetic energy."* In the case of a sling with a per- 

 fectly elastic cord whose elongation is proportional to the strain, the 

 tension or potential element does not " decrease as work is performed ; " 

 on the contrary, it increases directly with the work ; and whatever the 

 centrifugal velocity of the ball, the period of revolution remains con- 

 stant! The elasticity does not "expend itself in work." Vires ac- 

 quiriteundo. As this is simply the " laic of force "for this particular 

 tension, it has no relation whatever to " conservation," which pertains 

 entirely to " work." 



Again comparing gravitation to steam, whose pressure is weakened 

 by the absorption of molecular motion in the rising piston, the writer 

 repeats his questionings, " May not the same be equally true of gravity % 

 In fact, if gravity has a dynamical origin, it must hold equally true of 

 gravity and of heat." Again recurring to Mr. Waterston's conception 

 that " gravitation is a function of space," he would almost lead us to for- 

 get that " space cannot be a form of energy." Again endeavoring to fortify 

 his assumptions by scholastic dogmas, he reiterates, " the conception of 

 attraction does not represent the modus operandi of gravitation, because 

 attraction implies action at a distance, or in other words, that a thing 

 acts where it is not, which is just as impossible as that a thing can act 

 when it is not." Again affirming that " there is no necessity for forming 

 any conception of the cause of which it is the effect," his whole disserta- 

 tion appears impelled by the necessity to " represent the modus operandi 

 of gravitation." 



The general conclusion arrived at by Mr. Croll appears to be: "Grav- 

 ity in all probability is of the nature of an impact or a pressure. Some of 

 our most eminent physicists state that the force of gravity must either re- 

 sult from impact of ultramundane corpuscles, in some respects analogous 

 to that of the particles of a gas, (which has been found to be capable of 

 accounting for gaseous pressure,) or it must result from difference of 

 pressure in a substance continuously filling space, except where matter 

 displaces it. That gravity is a force of the nature of pressure is I think 

 beyond all doubt ; but that this pressure results from the impact of cor- 

 puscles, or from difference of pressure in a substance filling space, is 

 purely hypothetical. Why not assume it to be a force, without calling 

 in the aid of corpuscles or a medium filling space ?"| 



If gravity be an abstract force, not requiring " the aid of corpuscles or 



"Phil. Mag., 1876, vol. ii, p. 247. 



t The same isochronisin exists with the revolving or conical pendulum whose vertical 

 height is constant, or with one whose ball revolves in the surface of a paraboloid. 

 t Low eitat., p. 252. 



