75 
Steam Engine. 
die area of the piston, the pressure upon each inch of it^ 
the length of the stroke, and the number of strokes in any- 
assigned time : but the pressure upon any portion of the 
piston can only be ascertained by experiment and obser- 
vation, and that with didicul y, because of our uncertain 
methods of estimating friction and other species of resist- 
ance ; while judicious observations would with much 
less labour determine the work actually performed, either 
when the steam-engine works pumps, r gives motion to 
any kind of mills. The quantity of water raised by pumps 
in a given time may soon be estimated : and when the 
alternating motion of the steam-piston is converted into 
a rotatory one, the real effect the engine is capable of pro- 
ducing may be ascertained by observing the velocity with 
which a given weight is raised when suspended from the 
axle to which the rotation is first given. 
The usual method of estimating the effects of engines 
by what are called “ horse powers’’ must inevitably be 
very fallacious, unless all engineers could agree as to 
the quantity of work which they would arbitrarily assign 
to one horse, and in that case the term would manifestly 
be nugatory. It may also be observed, that in determin- 
ing the comparative value of different steam-engines, it is 
not sufficient to compare the quantities of work each will 
perform in equal times ; for the ex pence of erection, the 
probability of repairs being more or less frequent on ac- 
count of- the complexness or simplicity of construction^ 
and the quantities of fuel consumed by each, must like- 
wise be taken into the account.^ 
In giving a clear explanation of the principle of this 
machine, (Watt’s) it will be necessary to deviate from the 
* Boulton and Watt hav€ fixed and determined what they mean 
by a horse-power, viz. a power that will raise SSOOOlbs. one foot 
high in a minute, at a medium rate of working the engine. 
X. ,.C. 
