DYNAMICS. 
Thus, we find, that a pound of gold and a diminishing tendency ; but as it may he 
pound of lead, though apparently solid, give held in equilibrio at any period of its ex- 
very different weights within the same bulk. 
The former is said to be more dense, while 
the latter is said to be more rare ; density 
and rarity being opposite qualities. There- 
fore, if we could aiways ascertain the num- 
ber of atoms, or of minute parts, contained 
in a mass, we should be able to appreciate 
the density ; because, under the certainty 
that, in proportion as more or less atoms 
are comprised within a given space, so 
must the mass be more or less dense ; i. e. 
heavier, or lighter. 
In computing density we therefore take 
the rectangle of the mass, and, ascertain- 
ing the dimensions, discover by its weight 
how many atoms, or particles, it contains. 
Hence is derived our table of specific gra- 
vities, or the comparative weights of vari- 
ous bodies of unequal densities. (See Hy- 
DiiosTATics.) From this it will be seen, 
that bodies of similar substance and form, 
but differing in bulk, are to each other in 
proportion to the magnitude of their re- 
spective masses ; while, on the other hand, 
bodies of similar form, and equal in bulk, 
are, to each other, in proportion to their 
respective densities. 
Forces are considered according to the 
quantities of motion they are capable of 
producing ; but, as we cannot measure 
those forces, we are under the necessity of 
ascertaining the power by means of the 
effect. Thus we can correctly ascertain 
the force of gunpowder, by tlie effect pro- 
duced by the shot ; or we can fully explain 
the force with which tlie spring of a watch 
acts, by finding what resistance it is capa- 
ble of overcoming ; or we may compute the 
strength of a horse, by witnessing the 
weight he can draw. But it mast be obvi- 
ous, we could not discover per se, either 
the strength of the gunpowder, the elastic 
powers of the spring, or the muscular vigour 
of the animal. 
Force and velocity are, in fact, synoni- 
mous terms ; for the impetus given to the 
shot fired from a cannon, estimated by the 
Bulk of the shot, and the distance to which 
it may be projected, or tlie impression it 
may make on an opposing object, com- 
pletely supply the result of our reseat ch ; 
remarking, that, tliis being a diminishing 
force, its action will be strongest at the 
moment of expulsion, and gradually less as 
it recedes from the origin of motion, until 
it finally acquires a state of- rest. The 
spring in a measure partakes of the same 
ertion, it cannot be classed with the for- 
mer, though, rigidly speaking, it is assu- 
redly a diminishing force ; for as we see in 
clocks and watches, springs will in due time 
arrive at a state of rest, or inaction. The 
animal power is subject to so many anoma- 
lies, that it is next to impossible to treat of 
it with any strict adherence to calculation ; 
because, in so doing, v>'e are compelled to 
banish what we know to be the effects of 
labour, and to consider the power as always 
equal, and always maintaining the same 
physical ability. Here, indeed, we find 
theorists generally proceeding upon a wrong 
basis ; and, of course, rarely correct in 
their conclusions. We find them estimating 
the powers of horses, &c. as though their 
limbs had no flexibility, their muscles no 
relaxation, and as if their shoulders were 
insensible to pressure ; in fact, they gene- 
rally consider the animal as a fulcrum of 
wood, iron, &c. The absurdity of such a 
calculation must be obvious. 
But to proceed. We consider force to 
be either equal (or permanent), accelerat- 
ing (or gaining in power), or diminishing 
(or losing in power). Thus, the motion of 
a well-regulated clock may be considered 
as an equal force ; because, in equal periods, 
it proceeds over equal spaces. A weight 
falling from a height is an accelerating 
force ; because it gradually accumulates ve- 
locity in proportion to the space through 
which it falls ; and a shot fired from a can- 
non is a diminishing force ; because it con- 
stantly and gradually loses velocity, until, 
at length, it ceases to move. Dissimilar 
bodies wall move through the same space in 
exact ratio with tlieir own squares, and 
their relative unpulses ; but if two bodies, 
of equal bulk and density, be set in motion 
oppositely, by the same momentum, or 
power, they will hold each other in equi- 
librio : and if two such bodies so acted up- 
on should meet, they will mutually obstract 
each other’s progress. Of this we may 
frequently see instances in the game of bil- 
liards. But if two bodies of different den- 
sity be acted upon by forces that corre- 
spond with their masses respectively, the 
greater will overcome the lesser ; as will 
also a body impelled by a greater force than 
one of equal density, to which it may come 
in opposition. Thus, a pistol shot meeting 
a fives’ ball, will cause it to deviate from its 
course, or to recede. 
Uniform, or perfectly equal, motion, does 
Q q 2 
