MEASURE OF MOVING FORCE. 149 
The work done on a body, passing from a ve- 
locity v, answering tos =o, to any other velocity v is 
w=444(v'-v?), 
where the upper or lower sign is taken according 
as the velocity is accelerated or retarded. 
For important practical examples on this sub- 
ject, see ‘ Additions et Développements,” in the 
«‘ Mecanique Industrielle,” of M. Poncelet; and 
the Rev. Mr. Moseley’s “Mechanics of Engi- 
neering and Architecture.” The former writer, 
however, estimating the work necessary to over- 
come the inertia, and give the requisite velocity 
to the body p; and the latter regarding half the 
vis viva as work accumulated in the body, through 
its acquired velocity, and which work it is capa- 
ble of reproducing upon any body retarding its 
progress. 
WORK DONE BY MACHINES. 
In the “ Addition,” at the end of the second 
volume of Poisson’s “ Mécanique,” published in 
1833, there are given some very profound illus- 
trations of the use of the principle of living forces, 
(the vis viva) in calculating the effects of machines 
