kind, diminished. This hypothesis, then, seems never to have 
found a patron. But the other mediate view, that gravity is 
the result of propulsion, and that bodies and atoms aie pus e 
and driven together by pressure or impact from behind or 
beyond, has been a very frequent view. Newton inclines to 
it in his 21st Query. But in Query 28 he leans, I think, just 
as plainly to the opposite notion, that gravity is one ot two or 
three ultimate principles, of which cohesive force is another, 
which enter into the defining essence of matter, or “by which 
the things themselves are formed/’ 
Of this general view, that gravitation results from ethereal 
impact or pressure, there have been three varieties. First, that 
of Le Sage, that it depends on the impact of ultra-mundane 
corpuscles, flying in streams in all directions through space. 
He conceives them to come from beyond the limits of the 
known universe, and to produce attraction by impact on the 
molecules of matter, each screening its neighbour from some 
part or fraction of this celestial bombardment. A most 
grotesque machinery for securing the desired result ! But 
there is a plain and fundamental objection. II the molecules 
of matter are perfectly elastic to their etherial assailants, the 
differential effect would cease, and the action be equal on all 
sides. If their motion is quenched after the impact, the energy 
thus transferred from the ether to the matter on which it 
impinges must raise the whole universe to a white heat in a 
fow seconds. 
A second theory, hinted at, rather than proposed, is of this 
kind. “ If we suppose all space filled with a uniform, incom- 
pressible fluid, and that material bodies are always generating 
and emitting this fluid at a constant rate, the fluid flowing off 
to infinity, or else absorbing and annihilating it, the fluid 
flowing in from infinite space, the result would be an attractive 
tendency between any two bodies as the inverse square. . On 
this suggestion of Sir W. Thomson, Professor Maxwell justly 
observes, that such a hypothesis, ot a fluid constantly flowing 
out with no source of supply, or flowing in without any escape, 
is so contrary to all experience that it cannot be called, an 
explanation. But, with all deference to two mathematicians 
so eminent, I believe that the hypothesis is self-contradictory 
and impossible. If each particle of matter is surrounded by 
a plenum, nothing could flow out of it, for no room would be left 
into which it could flow. If by a fluid not a plenum, but 
homogeneous, as the hypothesis requires, it must cease to 
be homogeneous from the first moment when the outflow 
began. 
