48 Proceedings of Royal Society of Edinburgh. [sess. 
ether, or its resistance to motion ; and this, as we have already 
seen, is just balanced by the increased centrifugal tendency of the 
particles consequent on their increased velocity induced by the 
increased pressure.* We are now in a position to see how a body 
like the earth, bombarded as she is on every side by an infinity of 
radiations from an almost infinite universe of centres of energy 
(suns), can move with a great velocity through, certainly, a sub- 
stance of some kind (the ether), let it be dense or rare, without the 
slightest sign of retardation. 
Granted that she is formed of atoms of the nature above 
described, no one competent to give an opinion, I think, can deny 
that she, or any body like her, can go on so moving to all eternity 
without the slightest loss of motion. As a last word on this point,, 
let it be remembered that when resistance to motion generates a 
difference in pressure , the resistance is self -compensatory, and 
motion is neither destroyed nor generated ; but where a difference in 
pressure exists , not generated by motion , new motion is generated , 
and in the direction of least pressure. 
Before concluding this part of my subject, there is a certain 
peculiarity arising out of my theory which I should like to point 
out to my readers. It is, that owing to the radiations producing 
the pressure not being propagated instantaneously, but with the 
velocity of light, the centre of the earth’s motion cannot be sit- 
* I am anxious that this point should be thoroughly grasped ; so, at the risk 
of some repetition, I will go over it again in other words. Well, then, it 
seems clear that, granted the radiations producing pressure equal from all 
directions, if a body moved in any direction it must move against them, and 
if they have any influence on it they must, apparently, retard it, for the 
motion of the body added to that of the radiations opposing it strengthens 
those radiations, and weakens those following the body up behind, so that, 
apparently, nothing can save it from retardation. But owing to the 
constitution of my theoretical atoms no retardation, so far as the strengthen- 
ing of the radiation is concerned, could take place, for the increased pressure 
on the front of each atom would tend to increase the motion of the particles 
of which it is composed, and which increase would be neutralised by the 
decrease of pressure on the rear due to the diminished radiations from this 
direction, so that, on the whole, the mean motion of the particles of each 
atom would not be altered. This clears the way so far as the radiations 
are concerned, 'provided they are equal from every direction ; if they are not, 
it seems clear that the body must drift in the direction from which the 
diminished radiation reaches it, and with a velocity proportional to that 
diminished radiation. 
