258 
MOVING FORCE. 
Cases of diffi- portion of space, no motion can be produced. If it be clear 
movinj force, action of pressure through a portion of space, it is not less ob- 
vious, that the mechanical compression, or the mechanical 
separation of the parts of a mass of matter, must be produced 
by the same means j and when we speak of the resistance of 
inertia in one case, or of that of repulsion or cohesion in the ) 
other, we only mean, that the exertion of pressure through 
some portion of space is necessary to overcome the resistance i 
in either case. Although we suppose the resistance in the i 
different cases to proceed from different causes, we find no dif- ] 
ference in the means by which the resistance is to be overcome ; l 
and by taking the pressure conjointly with the space through t 
which it acts, we obtain a common measure for this description i 
of force. 
When we speak, therefore, of the force by which the motion j 
or the change of figure, of a mass of matter is produced, we c 
mean something more than simple pressure balanced by pres- 1 
sure 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 5 
can be produced unless the pressure act through some portion ^ 
of space. If the pressure be increased in the same ratio that n 
the space through which it acts is diminished, or vice versa, the , 
same effect will still be produced. The space, therefore, com- j, 
pensates for the pressure, and the pressure for the space ; and, 
when taken together, they constitute a determinate measurable jj 
quantity of moving force, capable of producing effects of vari- < 
ous kinds, but in determinate quantities, which are always ^ 
proportional to the moving forces by which they are pro- |j 
duced, jj 
The term force is often indiscriminately used to signify sim- ^ 
pie 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, (meaning quiescent pressure) and the 
force of a body in motion," are very common expressions. 
But thesQ two descriptions of force are as different in kind, as 
culty in the 
doctrines of 
that the motion of a mass of matter must be produced by the 
linee 
