in different forms of the Air-E’ngine. 173 
ing, may, in theory, be taken up again by a regenerator; but it 
does not follow that all may be again restored to the air. In 
order to save all the heat, the regenerator should be capable of 
reducing the temperature, at the close of the stroke, to t,,; but 
all which it absorbs between v and t,, will be lost. Moreover, a 
regenerator cannot maintain a temperature lower than v on its 
coldest side, so that it will be incapable of alone depressing the 
temperature sufficiently. All the heat taken in cooling between 
the limits just named, is invariably lost. 
There is, of course, in this form of engine, as in that which 
employs compression cylinders, a point beyond which a regenera- 
tor would be of no avail. This contrivance is confined in its 
efficiency, to the limits of temperature, t, and 7’. If we put these 
temperatures equal to each other, we shall have 
z 1 
sax igre s yr : v’\7-1 My ihe ee 5 ie 
ee (7) ee =1(7) “i (-) a 
Thus, in case that t” is double of t,, which will be approxi- 
mately the case in practice, there will be no advantage derived 
from a regenerator, when the expansion exceeds, 24V; but as 
this is beyond the limit of maximum power, the limitation is of 
little practical importance. 
he difficulty of employing a regenerator with thoroughness 
at all, is a more serious disadvantage. Stirling was able 
but a portion of the air through this contrivance: and, as a gen- 
eral rule, where the working piston is in direct contact with the 
air during the heating process, a certain portion of the mass must 
escape heating or refrigeration, or must be very imperfectly af- 
fected. Moveable regenerators, to take the place of Stirling’s 
Plungers, have been suggested by several persons; but besides 
that their weight would be an objection, they would be less easily 
ept down in temperature on the cold side, while it does not ap- 
pear that they have any decided advantage over fixed ones 
es. 
_ There is also great difficulty in applying furnace heat to the 
air in these engines. This was one of Stirling’s most serious 
troubles ; and mainly in consequence of this fact, it 1s probable 
that no new attempt will be made to construct an engine strictly 
on the principle now under consideration. ; 
By a modification of the principle, however, and by employing 
‘wo supply (or heating) cylinders, in aid of each working cylin- 
der, in one of which the air is in preparation, while in the other 
tls expanding into the working cylinder beneath the piston, an 
approach may be made to a realization of the degree of power 
Which theory indicates. : 
Those who have turned their attention to the planning of en- 
&ines without compression cylinders, have done so chiefly for the 
Sake of getting rid of what has appeared to them a great evil, in 

