, 260 
MOVING FORCE. 
Cuses of diffi terms need not be confounded, and if 
do'^f'”^ tlip 7noyi»^ybrfg were defined to be “moving pressure producing 
moving force change of velocity, or change of figure in masses of matter,” 
it could not be easily misunderstood. For, if the moving force 
be estimated by the ^changes which it produces, the space 
' ' through which the pressure acts, as well as the pressure, must 
be taken into the account. In the above definition it is neces- 
sary to adopt the expression change of velocity,” in prefer- 
ence to “ change of motion because change of direction is 
included in change of motion, and change of direction cannot 
be estimated by the pressure combined with the space without 
reference to the time. The centripetal force which retains a 
body in a circular orbit, is similar to quiescent pressure j the 
pressure at the centre moves through no space, and therefore 
there is no change of velocity j but if the revolving body 
approach or recede from the centre, any given space, the pres- 
sure moves through the same portion of space, and a corres- 
ponding change of velocity is produced Excepting change 
of direction, however, the above definition and measure of 
moving Jorce apply to every case of moving pressure of which 
we have any experience. 
The pressure taken together w'ith the time of its direct action, 
bears a constant relation to an important class of the pheno- 
^ mena of moving force producing motion in masses of matter. 
But when the pressure is applied indirectly by levers, or other 
means, or when a change of figure is produced, the velocity 
of tlie pressure being different from that of the mass which 
is moved, this relation is no longer preserved. In cases of that 
description, the sum of the changes produced by the moving 
force, is not in any constant ratio to the time of its action. If 
this statement be correct, the relation between the effects of a 
moving force and the time of its action, cannot be reduced to 
a general formula. It can only be considered as an individual 
character or property of one class of the phenomena of moving 
force, a property of great importance, no doubt, but still not 
a general property. The duration, therefore, of a moving 
force 
