Moving Force. 223 
the pressure conjointly with the space through 
which it acts, we obtain a common measure 
for this description of force. 
When we speak, therefore, of the force by 
which the motion, or the change of figure, of 
a mass of matter is produced, we mean some- 
thing more than simple pressure balanced by 
pressure, relatively at rest. In the latter case 
we have to consider only the pressures as they 
are balanced by each other, without any 
reference to motion. But in the former case 
no effect can be produced unless the pressure 
act through some portion of space.—If the 
pressure be increased in the same ratio that 
the space through which it acts is diminished, 
or vice versa, the same effect will still be 
produced. The space, therefore, compensates 
for the pressure, and the pressure for the 
space ; and when taken together, they consti- 
tute a determinate measurable quantity of 
moving force, capable of producing effects of 
various kinds, but in determinate quantities 
which are always proportional to the moving 
forces by which they are produced. 
The term force is often indiscriminately 
used to signify simple pressure, as well as to 
denote the compound quantity of force by 
which the motion of a body is produced.— 
The “ force of gravity” for example, (mean- 
