226 On the Measure of 
easily misunderstood. For, if the moving 
force be estimated by the changes which it 
produces, the space through which the pres- 
sure acts, as well as the pressure, must be 
taken into the account. In the above defini- 
tion it is necessary to adopt the expression 
“ change of velocity’? in preference to 
“ change of motion;” because change of 
direction is included in, change of motion; 
and change of direction cannot be estimated 
by tlie pressure combined with the space with- 
out reference to the time. The centripetal 
force which retains a body in a circular orbit, 
is similar to quiescent pressure ;—the pressure 
at the centre moves through no space, and 
therefore there is no change of velocity ; but 
if the revolving body approach or recede from 
the centre, any given space, the pressure 
moves through the same portion of space, and 
a corresponding change of velocity is produced. 
Excepting change of direction, however, the 
above definition and measure of moving force 
apply to every case of moving pressure of 
which we have any experience. 
The pressure taken together with the time 
of its direct action, bears a constant relation 
to an important class of the phenomena of 
moving force producing motion in masses of — 
matter. . But when the pressure is applied 
