The Vehicle of Force . 
615 
1880.] 
but also equal in speed. Although the opposed attraftions 
permit the particle to retain its general direction of move- 
ment, they tend to drag it back when its speed becomes too 
great, and to drag it forward when its speed becomes de- 
creased, until equality of speed is attained. The forces 
which oppose and neutralise each other direCtly aid each 
other transversely, so that, while the direction of motion 
remains unchanged, its speed is controlled, the constant 
tendency being to produce equality of speed in all similarly 
moving particles. 
Such is the influence at work in radiant impulses. Every 
new motion given to a particle exerts this influence on all 
neighbouring particles, and is transferred from particle to 
particle with wonderful rapidity. Its primary tendency is 
to drag neighbouring particles towards itself; but if this 
tendency is resisted through a similar drag in the opposite 
direction, it can only exercise its secondary tendency to drag 
the adjoining particles into equality of speed, and to yield 
to a reverse drag from them, the motive energy lost by the 
one particle being gained by the other. But it is only to 
the origin of this effeCt, not to its details, that we wish to 
here refer. 
There remains another force agency to be considered— 
the generally diffused and unvarying attractive force of 
gravitation. Investigation has hitherto been principally 
directed to the effects, not to the origin of this force. Yet 
it seems as if, under the above hypothesis, gravitation may 
receive a definite place among the modes of motion. 
The gravitative force of a particle has nothing to do with 
its fugitive special motions, which influence its behaviour 
towards its neighbours, but which vary in a manner to 
neutralise their influence on the great attracting masses of 
the universe. Its special motions, indeed, are but tempo- 
rary deviations from its general cosmical motions, and the 
tendency to resume the normal state causes any deviation 
in one direction to be balanced by an equal opposite devia- 
tion, so that all effects in one direction are neutralised by 
equal effects in the opposite direction. But these cosmical 
motions may have a fixed and persistent gravitative in- 
fluence. Thus the movements of the particles of the earth 
in its orbital revolution constitute a vigorous momentum, 
which should yield some degree of attractive influence upon 
the similarly rotating sun. In like manner the rotation of 
the sun upon its axis should yield a like attraction upon the 
similarly moving planets. The axial rotation of the earth 
should also create an attraction between its parallel lines of 
