1G 
over this ground on the present occasion; but a few items of 
evidence, which more especially appear to strengthen the main 
argument, I propose to inti’oduce here. 
(1.) In sec. 5, reference has already been made to the hypo- 
theses of the Theoiy of Universal Gravitation, which I shall now 
enunciate again for the purpose of expressing the second one 
in more definite terms, (a.) The force of gravity varies ac- 
cording to the law of the inverse square of the distance; (b.) 
it is universal as to the extent of its operation, and emanates 
from evemy elementary portion of visible and tangible substances. 
It is not assumed, in the theoretical calculations, that the force 
emanates from every atom, inasmuch as a vast number of 
atoms, in a state of aggregation, may be supposed to be con- 
tained in the space-element usually adopted in calculating the 
effects produced by the force which emanates from a given 
substance of given finite dimensions. Now as these two 
hypotheses are quantitatively expressed, it is a necessary 
consequence, according to the principles of our philosophy, 
that they should be deducible from its a 'priori hypotheses. 
I have, in fact, shown, on the supposition that every atom is 
a centre of ethereal vibrations by reason of the reaction at 
its surface, that the undulations, resulting from the compo- 
sition of the minor undulations propagated from all the atoms 
of a given small element, are capable of acting as an accelera- 
tive force on a distant atom, attracting it towards the element, 
and that this attractive force varies inversely as the square of 
the distance from the centre of the element. The universality 
of the force follows from the hypothesis of the unlimited 
extent of ether. In my early researches, I could not decide 
whether or not the fact of the equal acceleration of all bodies 
by the force of gravity was due to the elements being com- 
posed of atoms all of the same size; but at length I succeeded 
in demon strating on hydrodynamical principles that the gravity- 
undulations had the effect of accelerating equally atoms of 
different sizes. (The investigations here referred to are given 
in an article on the Hydrodynamical Theory of Attractive and 
Repulsive forces, contained in the Number of the Philo- 
sophical Magazine for September, 187G.) 
(2.) By a well-known experiment Gauss proved that the 
action of a large magnet, having its axis fixed, upon a small 
one restricted to oscillate about its middle point fixed, in a 
plane passing through the axis of the other, is, for the same 
distance between the middle points of the magnets, twice as 
great when the axis of the largo magnet is directed towards 
the middle point of the small one, as when the axis of the latter 
is directed towards the middle point of the large one, and that 
