SECTION VIII. 
OF EQUALIZING THE ACTION, REGULATING THE POWER, MEASURING 
THE USEFUL EFFECT, AND MANAGING THE STEAM ENGINE. 
534. The action of a steam engine is variable ; consequently, when an equable 
motion is necessary, its action must be equalized. It may also be employed in 
one hour to overcome a small resistance, and in another to overcome a conside 
rable one; therefore, the means of regulating the power to the work should be 
provided: we have also to consider certain methods which may be made sub 
servient to ascertaining the useful effect of an engine after it is erected, or, 
in the language of technical men, its performance ; and, lastly, the mode of 
managing the generation of steam, and the working of an engine. 
Of Equalizing the Action of Steam Engines. 
535. An equable motion is desirable in almost every kind of machine, it 
being strained much more by an irregular desultory one, as well as the fabric 
that supports it, than when the motion is equable. The strength of the machine 
must be adapted to the greatest strains that occur, but the quantity of work done 
is equivalent to the mean action only, and more is not performed by a desultory 
motion, than by one at a mean rate and uniform. There are two modes used 
for equalizing the action of an engine, which we propose to describe. The one 
is by the fly wheel, the other by a counter-weight. 
536. Of the fly wheel. A fly wheel is a wheel with a heavy rim which 
absorbs the surplus force at one part of the action, to distribute it again when the 
action is deficient; it has been aptly compared b}^ Professor Leslie, to “ a reservoir 
which collects the intermitting currents, and sends forth a regular stream.” 1 To 
equalize a motion which is subject to variation at each reciprocation in the steam 
1 Natural Philosophy, vol. i. p. 152.