KINETIC THEORIES OF GRAVITATION. 255 



may be equal, either has on the other side of it a mass of greater size, 

 or at a greater distance than the other, it is evident that the mutual 

 pressures of these two equal masses will under such conditions, be un- 

 equal, and hence as in the first case, they will approach. It is also 

 evident that a body may thus cause the approach to itself of another 

 body, whatever the number of interposed bodies. Thus if the concep- 

 tion of atoms is applied to the unequal and unequally-placed bodies of 

 such a world as that presented to us, the law of universal attraction 

 follows, and gravity is mechanically explained, that is, is referred to a me- 

 chanical conception. But it must be understood that the above propo- 

 sition is given rather to show that as an actual law, universal attraction 

 may be deduced from the theoretical conception of universal repulsion, 

 than with any pretension to its being the best attainable form of an ex- 

 planation of the law. It may however be remarked that such an expla- 

 nation is in accordance with the chief characteristics of the force of 

 gravity; it is not polar, and it seems to be so far different in kind from 

 other physical forces that it is not interchangeable with them, as they 

 are among each other; for the attraction of gravity is thus referred to 

 difference of mass, either between the two attracted bodies or in the 

 systems of which they are parts."* 



In a second article, "On the Principles of Energetics," Mr. Glennie 

 proceeds: u As force is thus conceived, not as an absolute entity acting 

 upon matter, but as a condition of the parts of matter itself, and as a 

 condition determined by the relative masses and distances of these parts, 

 any valid hypothesis of a force or of a motion to account for any set of 

 phenomena is thus seen to imply an assertion as to relative masses and 

 distances which can be more or less readily submitted to experiment or 

 observation and analysis. . . . 



" The condition of the begiuning of motion is a difference of pressure on 

 the body that begins to move; the condition of a uniform continuous 

 motion is a neutralization of the resisting pressure; the condition of an 

 accelerated continuous motion is a uniform or varying resisting [effect- 

 ive 1\ pressure." t 



In a subsequent paper, in continuation of the last, the writer thus re- 

 sumes his statement: "Here more clearly to express the idea in con- 

 trast with the fundamental hypothesis of Professor Challis, an atom may 

 be defined as a center of an emanating elastic aether, the pressure of which 

 is directly as the mass of its center, and the form of which depends on 

 the relative pressures of surrounding atoms. Thus if you will, matter 

 may be said to be made up of particles in an elastic rether. But that 

 a?ther is not a uniform circumambient fluid, but made up of the mutually 

 determining aethers (if you wish to give the outer part of the atom a 

 special name) emanating from the central particles. And these central 

 particles are nothing but what (endeavoring to make my theory clear by 



* L. E. D. Phil. Mag., January, 1861, vol. xxi, pp. 41-45. 

 t Loco citat., April number, p. 27G. 



