44 Proceedings of Royal Society of Edinburgh. [sess. 
produce the pressure in the medium ; consequently the space 
separating them would he deficient in pressure, with the result 
that, owing to the superiority of pressure in the space outside of 
them over that between them, they would be pressed together. 
The one would screen the other, in the same way that a screen 
placed in front of a fire would prevent the radiations of the fire 
from pursuing the paths in which they originally set out. In such 
a case the screen in itself becomes a hot body, and, as such, dis- 
tributes its radiations impartially in every direction ; so much of 
them being sent back into the fire again as into the screened space, 
and quite as much in every other direction as into this space, with 
the result that the latter is kept cool. 
Now, then, take such a body as the sun,* and another body 
such as the earth, and consider the condition of the medium in 
the vicinity of each. In the vicinity of the sun the decrement 
of pressure would be very rapid, and in that of the earth not nearly 
so rapid. It would be as though the earth were placed on the 
summit of an exceedingly high, steep mountain, and the sun on a 
gently sloping eminence, and permitted to roll towards one another, 
the motion of each would be proportional to the steepness of its 
path. Of course their paths must be supposed to grow steeper 
and steeper the nearer they approach one another, inversely as the 
square of their distance apart, and directly as their masses. This, 
then, answers all the requirements of gravity. Every mass in the 
universe is making momentary use of a quantity of force or pressure 
out of the medium surrounding it, and which quantity is propor- 
tional to each mass ; each mass screens every other from the radi- 
ations which cause the pressure, with the result that the spaces 
between them are kept free from these radiations in proportion to 
the screening masses. Consequently, the pressure in the screened 
spaces being weaker than that outside them, the bodies tend to 
approach one another in proportion to the difference of pressure. 
Now, a few words as to the supposed instantaneous action of 
gravity. It is objected to all hydro -dynamical theories of gravi- 
tation that they are “obnoxious to the fatal criticism of Arago ” : — 
* It must be borne in mind that it is only the radiations causing pressure 
which would be effective here — not the sun’s waves. These would only be 
effective after dwindling to their lowest form. 
