ai 
APRIL 18, 1884. ] 
which the student may perform in order to be- 
come thoroughly acquainted with that property 
of matter which he calls inertia. I shall de- 
seribe an additional experiment, for I find that 
the difficulty is not merely one of words. There 
are many people who do not recognize the phys- 
ical facts to be dealt with. 
Take a heavy weight, fifty pounds let us 
say, and suspend it by a long cord. To the 
weight thus hanging attach another cord, strong 
enough to sustain the fifty pounds. By means 
of this latter cord give a sharp horizontal pull 
to the weight. The cord is broken, while the 
weight hardly moves, — is left slightly swing- 
ing. Is it possible for any one to try this ex- 
periment, and not recognize that we have to do 
here with something more than the inability of 
matter to set itself in motion? Evidently we 
encountered a resistance in setting the body in 
motion. Whence came that resistance? Not 
from gravity: the pull was horizontal; and, 
moreover, the cord we have broken would have 
served to lift the weight. Assuredly not from 
friction, or resistance of the air. We are 
driven to the conclusion that matter possesses 
a property in virtue of which it offers resist- 
ance to an agency which is setting it in motion. 
We should find, too, by experiment, that mat- 
ter offers a similar resistance when its motion 
is being changed in any way, either in magni- 
tude orin direction. ‘This property of matter, 
which is much more than the mere inability to 
set itself in motion, is what Maxwell, Thomson, 
and Tait call inertia. 
Now, we must distinguish very carefully be- 
tween inertia itself, a property of matter, and 
the resistance which matter can exert in virtue 
of that property, somewhat as we must distin- 
guish between a man’s strength (that is, the 
property in virtue of which he can exert force) 
and the force which he may be actually exerting 
at any time. 
Returning to experiment, let us attach to 
our fifty-pound weight a weak thread, capable 
of sustaining a few ounces. Pull gently and 
steadily in a horizontal direction upon this 
thread. A resistance is felt, to be sure; but 
the weight is moved perceptibly in the direc- 
tion of the pull, and acquires, perhaps, a greater 
velocity than we succeeded in giving to it by a 
pull which broke the cord previously used. 
This experiment proves that the resistance 
which a given body can, in virtue of its inertia, 
offer to an agency which is setting it in motion 
(and it would be the same for any change in 
its motion), is a variable quantity —let us 
leave the statement unfinished for a moment, 
SCIENCE. 
483 
while we look for the conditions and the law 
of this variation. When the stout cord was 
broken in pulling at the hanging weight, the 
latter acquired a small velocity, it is true; but 
it acquired that velocity in a very short time, 
a fraction of a second. When pulled by the 
thread, the weight acquired a somewhat greater 
velocity, it may be; but a much longer time 
was occupied in the change. The exact quan- 
titative law, which can be determined by exper- 
iment with such apparatus as, for instance, 
Atwood’s machine, is expressed by complet- 
ing the interrupted statement in the follow- 
ing words :— being proportional to the rate 
at which the agency is changing the body’s 
motion. 
This definite law being recognized, there 
should be an end of the current vague attempts 
at explaining such phenomena as, for example, 
that of a half-open door pierced by a cannon- 
ball without being shut. Text-books too fre- 
quently say, in such a connection, that ‘‘ masses 
of matter receive motion gradually and surren- 
der it gradually,’’ or that ‘‘ it requires time to 
impart motion to a body as a whole,’’ —state- 
ments from which the student is in danger of 
getting the idea, if indeed he gets any idea, 
that the time is required in order to draw 
things taut within the body, and get its parti- 
cles to acting upon each other, somewhat as it 
takes time and a succession of jerks to take 
up the slack of a freight-train while it is being 
started. 
Let us note again that the resistance which 
has just been considered is not the body’s in- 
ertia, but is merely the manifestation of that 
property. But if the manifestations of inertia, 
in the case of a given body, are so variable, how 
can we speak of inertia as a measurable quan- 
tity, as Maxwell does in the quotation already 
made from him? 
Suppose we take a certain body, and ascer- 
tain what force, reckoned in any units we 
please, is required to impart to this body a 
certain velocity in a certain time. Then we 
take a second body, and ascertain what force 
is required to give it the same velocity in the 
same time. The second force may be equal 
to, less than, or greater than, the first. Ifthe 
forces are equal, we may say that the two bodies 
have equal inertias. If the second force is » 
times the first, we may say that the second body 
has n times as much inertia as the first. This 
is comparison of inertias. If we wish for what 
is called measurement, we have merely to select 
some body, and agree to call its inertia the unit 
inertia. K.. Be Man, 
