S76 Steam Engine^ 
Description of an Engine for affording mechanical power 
from air expanded by heat ; by Sir George Cay ley ^ 
BarV^ 
To Mr. NICHOLSON. 
Sir, Brompton^ Sept, 25, 1807. 
I OBSERVED in your last vol. p. 368, that some 
experiments have been lately made in France upon air, 
expanded by heat, applied as a first mover for mechanic 
cal purposes. This idea, as you justly remark, is by no 
means new in this country ; yet I have not heard that any 
successful eKperiments have been made, exclusively upon 
this principle, in England, though you hint that some- 
thing promising has been accomplished relative to it. 
The subject is of much importance, as the steam en=» 
gine has hitherto proved too weighty and cumbrous for 
most purposes of locomotion ; whereas the expansion of 
air seems calculated to supply a mover free from these 
defects. Under this . impression I send you a sketch of 
an engine I projected upon this principle several years 
ago ; it was made on a considerable scale at Newcastle, 
though I must confess without success in the result, which 
I attributed to the imperfect manner in which it was exe«> 
cuted, the cylinders being made of sheet copper, and so 
irregular, as not to be rendered tolerably air tight by any 
packing of the piston, I think there can be no doubt 
that the scheme is practicable in some way or other ; and 
I conceive that the form of the engine here sketched will 
be the basis of whatever experience may prove to be ad- 
ditional requisite to perfection in the apparatus of the air 
engine. 
A and B, Plate 8 fig. 1, are two cylinders, placed one 
* I extract this paper merely to give notice of the idea express 
sed in it. I do not think it necessary to give the plates of a ma- 
chine that has not succeeded. T. C. 
