A. B. Quinby on Crank Motion. 321 
known, has no connexion with a loss of power in the steam- 
ne. sae 
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. he 
power of a steam-engine is the product of the elastic force of 
the steam employed and the surface of the piston upon which 
It acts. . 
This definition being admitted, it is easy to perceive that 
both the writers who have made the estimates, have com- 
mitted not only “a very great,” but a very egregious, blun- 
der, since they both have taken, not the elastic force of the 
steam employed, and the surface of the piston upon which it 
acts, but the quantity of coal consumed, for the measure of 
the power of the respective engines. 
Besides this blunder, it is further to be noticed, that both 
the writers in question have founded their inference on the 
presumption that the engines employed in drawing the matter 
out of the mines, are always uniformly and sufficiently loaded. 
But on this subject they have stated no evidence, or have re-~ 
ferred to no authority ; and it is more than probable that, in 
raising the matter out of the mines, the load applied in the 
buckets was neither uniform nor sufficient; and consequently 
“a blunder’ has, on this score, been committed, as well as 
in making the quantity of coal consumed to be the measure 
of the power of the respective engines. 
I shall now notice the words, “ As appears from the re- 
ports on the performance of the engines used at the mines in 
Cornwall.” 
thods in common practice.” 
In Tilloch’s Philosophical Magazine we have the whole 
series of reports on the performance of the engines used at 
the mines in Cornwall, by Messrs. fF. & J. Lean, com- 
mencing Aug. 1811, and ending Nov. 1818; and in these 
reports the case noticed by the writer of the article Steam 
Engine, Rees’ Cyclopedia, is not mentioned ; and if it was 
mentioned it would never !ead a scientific writer, who is com- 
petently acquainted with the steam-engine and with the 
V wt. 1X.—No. ¢ Aq 
