MEASURE OF MOVING FORCE. 153 
work produced by the machine. When the 
machine has attained to a permanent state, the 
quantity of work performed during a given time 
by the moving forces is not wholly represented 
by /=Qdq the effective part of the resisting 
work; for this is always less than the motive 
work /=Pdp by that which is done in overcom- 
ing friction, and other resistances. A machine is 
more perfect according as the effective work 
approaches to an equality with the motive work. 
As an example of imperfect machinery, in which 
the effective work is only a very small fraction of 
the motive work, M. Poisson adduces the machine 
formerly used to raise water from the Seine at 
Marly. For an account of this magnificent col- 
lection of injudiciously constructed mechanism, 
see ‘Bélidor, Architecture Hydraulique,” or, 
** Gregory’s Mechanics, vol. 2.” 
Professor Moseley, in his ‘‘ Mechanical Prin- 
ciples of Engineering and Architecture,” gives 
a different interpretation to the vis viva of a 
moving body, from that of M. Poncelet, before 
alluded to, feeling that the use of the term vis 
inertia, employed by that writer, and associated 
with the distinct idea of a force opposed to all 
change of motion, is attended with difficulties 
which it would be better to avoid. He has, 
Y 
