30 
inertia;, I have only to remark that if the thing itself be understood from 
sensation and experience, it matters not -whether it be called inertia or vis 
inertias. In secs. 15 and 21 of the paper before cited (Journal, V ol. XI. pp. 202 
and 204), I have endeavoured by a familiar instance to make intelligible the 
fact and the quality of inertia, and have given reasons for concluding that 
“ the reality of inertia as a quality pertaining to bodies is recognizable by a 
sense of personal effort.” Probably the feeling that inertia, although not 
an active force, is something to be overcome by force, and the natural infer- 
ence that what force overcomes is itself force, may have given rise to the 
expression vis inertice. I am aware that some eminent experimentalists have 
been indisposed to accept “ inertia ” as a philosophic term ; but the theo- 
retical calculator knows that he cannot proceed a step towards forming his 
equations of force without taking into account the intrinsic quality of matter 
which this term expresses. 
Thinking that it may be expected of me to advert to the discussion which 
took place between the Chairman and Mr. Challis relative to the quality of 
the force of gravitation, I beg to make the following remarks on that ques- 
tion. Let it be granted that the unit-measure of the gravitating force of any 
mass is “the action [? moving force of the mass] on a unit of matter at a 
unit of distance,” and that this measure is “ constant and uniform,” there 
still remains to be considered the noteworthy fact that the quantity of the 
gravitation of the same mass has to this standard measure a ratio which is 
different for every different distance from the mass. The circumstance of 
this variability in space is expressly adduced by Newton as the reason that 
gravity is not, as inertia is, an intrinsic quality of matter. This quantitative 
variation of gravity is precisely analogous to the difference of effect produced 
on the ear by the sound of a bell at different distances from the spot where 
it is sounded. In this instance we know that the variation arises from the 
sound being transmitted by the propagation of divergent waves of the air. 
Just so in the proposed theory of gravity, waves of the ether, superior in 
order of magnitude to those which produce heat or light, are supposed to 
emanate from all the parts of masses, and to produce an attraction varying 
in its effect on external bodies according to the law of the inverse square of 
the intervening distances. To make this argument good, it is necessary to 
prove that the vibrations of an elastic medium constituted like air of given 
temperature, are capable of drawing bodies towards the parts from which 
the propagated vibrations emanate. This I consider I have succeeded in 
doing in the communication which is referred to at the end of sec. 22 (1) of 
the present paper, as being contained in the Philosophical Magazine for 
September, 1876. The reasoning which conducts to this result depends 
essentially on the definition of the ether given in the third of the hypotheses 
enunciated in sec. 10. It is true, as Mr. Brooke has remarked, that, ac- 
cording to this view, bodies are “ pushed ” towards each other by the force of 
the hypothetical ether ; but it is not correct to say that this force “ does not 
depend on the bodies themselves,” inasmuch as the gravity-waves which 
produce the effect have their origin in the bodies. Mr. Challis justly urged 
