MEASURE OF MOVING FORCE. 155 
ments of the machine, or the work accumulated 
in them, and reproducible when a falling off occurs 
in the moving power. 
When the motion of the machine has become 
uniform, the last term of the preceding equation 
disappears, and we have 
as might be inferred from the principle of virtual 
velocites. 
The theorems previously given apply to ma- 
chines, subject to acceleration; but, in actual 
practice, it fortunately happens that we have sel- 
dom need to calculate their effects, except when 
their motion is uniform, or periodical as in the 
Steam Engine ; and, in these cases, the easy for- 
mula of virtual velocities, last given, is sufficient. 
There exists in every machine, a direct rela- 
tion between the following elements :—the work 
done upon the moving points, that expended upon 
the working points, that required to overcome 
the prejudicial resistances, and that accumulated 
in the moving elements of the machine ; Professor 
