46 Proceedings of Royal Society of Rdinburgh. |_sess. 
distance decreased, and unwound as it increased. Newton himself 
disclaimed the idea of attraction as an ultimate physical fact. In 
his third letter to Bentley he says : — “ It is inconceivable that 
inanimate brute matter should, without the mediation of something 
else which is not material, operate upon and affect other matter, 
without mutual contact, as it must do if gravitation, in the sense of 
Epicurus, be essential and inherent in it. And this is the reason 
why I desired you would not ascribe innate gravity to me. That 
gravity should be innate, inherent, and essential to matter, so that 
one body may act upon another at a distance through a vacuum, 
without the mediation of anything else by and through which 
their action may be conveyed from one to another, is to me so great 
an absurdity that I believe no man who has in philosophical 
matters a competent faculty of thinking can ever fall into it. 
Gravity must be caused by an agent acting constantly according to 
• certain laws ; but whether this agent be material or immaterial, I 
have left to the consideration of my readers.” 
If the action of the sun on the earth were a pulling or attract- 
ing action, then, unless it were perfectly instantaneous, the result 
pointed out by Arago, Proctor, and the rest would certainly follow ; 
but if gravity be caused by a pressing or pushing force, as in this 
theory, retardation, at first sight, instead of acceleration, would 
seem to result. For the radiations to which the pressure in this 
theory is attributed would, owing to the earth’s velocity in her 
orbit, meet her partly on her front instead of acting in direction of 
the sun, or at right angles to her path, and seemingly she ought to 
be retarded. Of course I mean that portion of the radiation which 
is effective in producing gravity, or that travelling in the sun’s 
direction ; this would meet her partly on her front, and apparently 
retard her ; but besides this, the radiations meeting her directly from 
the direction in which she was travelling being strengthened by 
ner motion, added to their own, ought apparently to retard her still 
more, for the radiations following her up behind being minus her 
motion, would be so much in defect. Apparently, then, nothing 
can save her from retardation. But before explaining this, it is 
first necessary to show how bodies can travel through the ether, 
• dense as I believe it to be, without loss of motion. 
I am now about to describe to my readers one of the most 
