36 Rejoiader to Mr. Quinby on Crank Moti. 
average of thirty-four millions of pounds one foot high; witli 
Watt's pumping engine, an average of twenty-eight millions 
ot 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 c . Mr. Quinby tells us that ‘* with, 
a view to clearness, and for the information of the writer et 
the article in the North Ameriean Review, I shall here give 
a a ete of the power of asteam-engine. ‘The power of 
steam-engine is the product of the elastic force " the 
stoam employed, and the surface [area I suppose] of. the 
iston 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 
obtaining from it the mean of the constantly 
yeuying : pressure. in. the cylinder.* But even if the force of 
steam in the c cylinder and in the boiler were exactly the 
same, during the ames the induction pipe is open, still from 
ihe constant variations of the intensity of the fire, constant 
observation .of a 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 
uired. But the best of the matter is, that Mr. Quinby’s 
depnition, introduced with no small flourish, is not suflicient 
for the end proposed. ‘The power of a steam-engine caa- 
not be determined by it, even for any instant of time, or 
‘during any part of the stroke, as he has neglected to notice 
ete nazetin 8 effect of the elastic fluid in the condenser 5 and 
“ defining’? the force to be the excess of the 
ae ‘of the steam upon one end of the piston, over the 
pressure of the elastic fluid epan the other end, which is the 
iieiecac oui tpi titulo t ebpci'tn Miike li bans OAD 
+95 egg dey seagate h the invention of it is ascribed to Mr. W: wake 
he seems foie Nate sotieteha with, te minmnendesined from the ccn- 
‘option ot cons a enapartivepurpones 1 ase eal 
