Dr. Hudson on an Error in Dr. Apjohn’s Formula, 21 
Now, in former numbers of this Journal*, I have shown 
that in 1811—1812, Captain Trevithick erected a single engine 
at Huel Prosper mine, in which steam of above 40 pounds 
pressure on the square inch was worked expansively. Mr. Farey 
observest that Mr. Woolf came to reside in Cornwall about 
the year 1813, and the “ first engines for pumping water from 
the mines were set up by him in 1814;” but these, he adds, - 
had two cylinders. I therefore repeat, that we do not owe 
this practice to Mr. Woolf, but to Captain Trevithick. But it 
has been already shown that this is only an extension of 
Mr. Watt’s practice of 1778. 
The advance in the duty of steam engines which has taken 
place in this county within the seven years last past, is princi- 
pally, if not entirely, due to Captain Grose; and is obtained 
mainly by the application of substances which transmit heat 
very slowly, to the surfaces of the portions of the apparatus 
containing dense steam. 
I remain, Gentlemen, 
Yours, &c. 
1, Morrab Place, Penzance, W. J. HEnNwoop. 
November 28, 1835. 
VI. Onan Error in Dr. Apjohn’s Formula for inferring the 
Specific Heats of dry Gases. By H. Hupson, M.D., 
M.R.LA. 
To the Editors of the Philosophical Magazine and Journal. 
GENTLEMEN, 
BEG to point out a serious error in the formula given (in 
p. 385 of your 7th volume) by Dr. Apjohn for ascertaining 
the specific heats of gases by their action on the “ evaporation 
thermometer.” I have shown (in your Journal for October 
last, same vol., p. 257,) that (taking the density of air at 212° 
under 30 pressure as unity) the density of the vapour of satura- 
tion at 7° is = wept * 13°75, consequently (the weight of 
*327958 
a cubic inch of air at 212° under 30 being nti E t) the 
weight of a cubic inch of the vapour of saturation at 2° i= 
of Grs. 
—/—_ x 327958 ; at of at t° be- 
usa * 3°27958; also the latent heat of vapour at ¢° be 
* Phil. Mag. and Annals, vol. vii. p. 323, March 1830, and vol. x. p. 98, 
August 1831. + Ibid., vol. viii. p. 308. 
f -327958 is the weight at 32° (Prout), and 1°375 the expansion be- 
tween 32° and 212° (Gay-Lussac). 
