36 Repoinder to Mr. Quinby on Crank Motiou. 
average of thirty-four milhons of pounds one foot high ; witi: 
Watt’s pumping engine, an average of twenty-eight millions 
of pounds one foot high nearly ; while with the rotary engine 
it never exceeds eleven millions of pounds raised one foot. 
‘as appears from the reports of the performance of the en- 
gines used at the mines in Cornwall.” [It happens also. that 
the consumption of coals is the only practicable measure 
by which the work of different engines, through any length 
of time, can be compared. Mr. Quinby tells us that ‘* with 
a view to clearness, and for the information of the writer of 
the article in the North American Review, I shall here give 
a definition of the power of a steam-engine. The power of 
a steam-engine is the product of the elastic force of the 
steam employed, and the surface [area I suppose] of the 
piston upon which it acts.” But every body knows that the 
pressure of the steam upon the boiler, is no certain indica- 
tion of the pressure in the cylinder, or upon the piston ; and 
although an instrument has been contrived for determining 
the pressure in the cylinder, yet it appears to have been 
rarely used, owing probably to its complicate structure and 
the trouble of obtaining from it the mean of the constantly 
varying pressure in the cylinder.* But even if the force of 
the steam in the cylinder and in the boiler were exactly the 
same, during the time the induction pipe is open. still from 
the constant variations of the intensity of the fire, constant 
observation of the gauge would be required to obtain the 
mean pressure ; nor would this be all, for a constant know- 
ledge of the perfection of the condensation, would also be 
required. But the best of the matter is, that Mr. Quinby’s 
definition, introduced with no small flourish, is not sufficient 
for the end proposed. The power of a steam-engine can- 
not be determined by it, even for any instant of time, or 
during any part of the stroke, as he has neglected to notice 
the negative effect of the elastic fluid in the condenser; and 
instead of “ defining’? the force to be the exeess of the 
pressure of the steam upon one end of the piston, over the 
pressure of the elastic fluid upon the other end, which is the 
** No account of this instrument appears to have been published until 
the year 1822, and although the invention of it is ascribed to Mr. Watt, 
he seems to have been satisfied with the measure derived from the con- 
sumption of coals for all comparative purposes. 
