INE 
neither can the rectilinear and uniform mo- 
tion of a body be changed without the ac- 
tion of a foreign cause. That want of ap- 
titude which bodies liaye, of producing in 
themselves a change in their actual state, is 
called inertia. Now it is known that a 
body, whose state' may be changed by the 
action of a foreign force, cannot give way 
to that effect, otherwise than by itself al- 
tering the state of that force; that is to 
say, by itself taking away a part of its mo- 
tion, It has hence been concluded, that 
the continuance of a body in its state of 
repose, or of uniform motion, was itself the 
effect of a real force which resided in that 
body ; and this force has been viewed, 
sometimes as a resistance in so far as it op- 
posed itself to the action of the other force, 
which changed the state of that body, and 
sometimes as an effort, in so far as it tended 
to carry with it the change in the slate of 
the other force. 
The celebrated Laplace has proposed a 
more precise and natural manner of con- 
templating inertia. To conceive in what it 
consists, suppose a body in motion to meet 
with a body at rest : it will commupicate 
to it a part of its motion; in such manner, 
that if the first have, for example, a mass 
double to that of the second, in which case 
its mass will be tw o-thirds of the sum of the 
masses, the velocity which it will retain will 
be also two-thirds of that which it had at 
first ; and as the other third which it has 
yielded to the second body employs itself 
upon a mass of only half the magnitude of 
the former, the two bodies will both have 
the same velocity after the shock. 
The effect of inertia is reduced, therefore, 
to the communication made by one of these 
bodies to the other, of a part of its motion ; 
and since this latter cannot receive, but in 
consequence of the other’s losing, this loss 
has been attributed to a resistance exercised 
by the body receiving the motion. But in 
the instance before us, it is very nearly as 
in the motion of an elastic fluid, contained 
in a vessel from which we would open a 
communication to another vessel winch 
should be empty , this fluid would intro- 
duce itself by its expansive force into the 
second vessel, until it became uniformly 
distributed in the capacities of the two 
vessels : in like manner a body when it 
strikes another does nothing else, if we 
may so express onrselves, than pour into 
this latter a part of its motion ; and there is 
no more reason to suppose a resistance in 
this case than in the examples we have just 
INF 
cited. It is true that when we strike with 
the hand a body at rest, or whose motion is 
less rapid than that of the hand, we imagine 
that we experience a resistance ; but the 
illusion proceeds from this, that the effect is 
the same with regard to the hand, as 
though it were at rest, and was struck by 
the body with a motion in a contrary di- 
rection. 
INFAMY, in law, which extends to for- 
gery, perjury, gross cheats, &c. disables a 
man to be a witness or a juror; but a par- 
don of crimes restores a person’s credit, to 
make him a good evidence. 
INFANCY, the first stage of life. Ip 
a medical and political view, extending 
from birth to about the seventh year. Like 
every other stage of life, it is subject to its 
peculiar diseases, even in the healthiest 
state of the constitution, and under the best 
and most natural controul. But from a 
too generally inherent debility produced by 
the common consequences of polished and 
fashionable life, added either to maternal 
neglect, or a superabundance of maternal 
assiduity and anxiety, it is also exposed to 
diseases of great variety and violence from 
which it would otherwise be exempt. 
The natural infirmity of infant life ex-? 
poses it at all times, and in all situations, to 
a mortality far exceeding that of any other 
stage : but from the powerful effect of such 
accessary causes, in conjunction with the 
impure air of crowded towns and cities, 
we are often called upon to contemplate 
this mortality, trebled or quadrupled be- 
yond its appropriate ratio ; and to behold 
more than half the natives of a place die 
within the narrow term of the first three 
years of life, instead of the greater part of 
them reaching the age of twenty-five or 
thirty, and, of course, living to become hus- 
bands and wives, and giving birth to gene- 
rations which are thus cut off along with 
themselves. 
What then are the best means of dimi- 
nishing this melancholy and sweeping mor- 
tality ? There cannot be a more important 
question either ip-qjedicine or in politics. 
To offer all that < Worthy of notice in the 
latter view, would be to engage in a much 
longer and a more speculative discussion 
than the limits of the present article, or 
even of the present work, would allow. We 
shall confine ourselves, therefore, in what 
we shall have further to advance upon the 
subject to the point of medical and domes- 
tic attention alone ; and shall beg leave. 
