many small engines do not convert into power more than 2 per cent. of
the potential energy contained in the coal.

At one time the steam-engine was threatened with serious rivalry by
the hot-air engine. About the year 1816 the Rev. Mr. Stirling, a
Scotch clergyman, invented one which a member of this Institute (Mr.
George Anderson) remembers to have seen still at work at Dundee. The
principle of it was that a quantity of air under pressure was moved by
a mass, called a "displacer," from the cold to the hot end of a large
vessel which was heated by a fire beneath and cooled by a current of
water above. The same air was alternately heated and cooled, expanded
and contracted; and by the difference of pressure moved the piston in
a working cylinder. In this arrangement the furnace was inefficient.
As only a small portion of heat reached the compressed air, the loss
by radiation was very great, and the wear and tear exceedingly heavy.
This system, with some modifications, was revived by Rankine,
Ericsson, Laubereau, Ryder, Buckett, and Bailey. Siemens employed a
similar system, only substituting steam for air. Another system,
originally proposed by Sir George Cayley, consisted in compressing by
a pump cold air which was subsequently passed partly through a
furnace, and, expanding, moved a larger piston at the same pressure;
and the difference of the areas of the pistons multiplied by the
pressure common to both represented the indicated power. This
principle was subsequently developed by a very able mechanic, Mr.
Wenham; but his engine never came much into favor. The only hot-air
engines at present in use are Ryder's, Buckett's, and Bailey's,
employed to a limited extent for small powers. I have not said
anything of the thermal principles involved in the construction of
these engines, as they are precisely the same as those affecting the
subject of the present paper.