MR. J, P. JOULE ON THE AIR-ENGINE. 
75 
Professor W. H. Miller has remarked that Moll’s experiments, when correctly 
reduced, give a velocity of sound equal to 332’475 metres per second in dry air at 32°. 
Hence he deduces 1’4]029 as the value of h. Calling it in round numbers r41, and 
the mechanical equivalent of heat 772, we obtain 0‘238944 as the value of the specific 
heat of air at constant pressure, a result sufficiently near the experimental determi- 
nation to sliow that the value of Tt, as deduced by Professor Miller, is much nearer 
the truth than that upon which the tables of the foregoing paper are founded. 
The values of h, as determined by the experiments of Desormes and Clement, 
Gay-Lussac and Welter, and Mr. Meikle, referred to in the note to page 67, are re- 
spectively only 1*354, 1*375, and 1*333. In these experiments a small portion of air 
having been withdrawn from a large receiver, the equilibrium was re-established by 
opening for an instant a large aperture communicating with the external air, -and 
then, after the receiver and its contents had regained their original temperature, the 
alteration of pressure, indicating the sudden rise of temperature which had taken 
place on the admission of the air, was noted. But it is obvious that the sudden ad- 
mission of the air wrnuld cause the development of sound, and that, a portion of the 
vis viva escaping in this form, the increase of temperature and the deduced ratio of 
the specific heats would be diminished accordingly. 
I subjoin Tables, similar to Tables I. and II., calculated from the data A:=l*41, 
and the specific heat of air at constant volume =0*169464, or at constant pressure 
=0*238944. 
In Table IV., the examples 9, 10 and 11 may be suggested to the notice of the 
practical engineer, the temperature of the receiver being in all those cases below that 
of redness. I may remind the reader that the Table is founded on the supposition 
that the air which enters the pump has 491° of temperature from the absolute zero, 
and that its pressure is 15 lbs. on the square inch. If this initial temperature be 
altered, the whole of the other temperatures in the Table must be altered in the same 
proportion, but the pressure, work and economical duty will remain unchanged. If 
the initial pressure be altered, all the other pressures and work will suffer a propor- 
tionate change, but the temperatures and economical duty will remain the same. The 
above are obvious deductions from the formulse on which the Tables are founded. 
Acton Square, Salford, 
March 20, 1852. 
