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Mechanical Theory of Heat to the Steam Engine. 208 
ther with as little danger as if it were a permanent gas. Ma- 
chines which employ the steam in this condition can unite many 
advantages of steam engines with those of air engines, and a 
practical result is therefore sooner to be expected from them, 
than from the air engines. 
second more volatile substance is applied, the interval (7’,— i) 
is made larger because 7’, is made lower. The idea has sug- 
gested itself in the same manner to increase the interval on the 
6. Besides the imperfection of the common steam engines 
just mentioned, which is founded in their nature itself, these ma- 
chines have many other defects, which are to be attributed more 
to their practical construction. ae, 
One of these has already been considered in the above devel- 
opments, and is comprised in equation (x), namely, that the ex- 
pansion cannot by any means be carried so far that the steam in 
the cylinder reaches the temperature of the condenser. If we 
take, for instance, the temperature of the boiler at 150°, and that 
of the condenser at 50°, we see from the table of § 16 that for this 
purpose the expansion must continue to 26 times the original 
volume, while in reality in consequence of many evils which 
occur in high expansions, we usually allow it to reach only 8 or 
4, and at the utmost, 10 times the volume. 
Two other defects, on the other hand, have been expressly 
excluded in what precedes, namely, in the first place that the 
pressure of the steam in one part of the cylinder is less than in 
the boiler, and in the other part greater than in the condenser— 
and secondly, the presence of the injurious space. 
We must therefore now enlarge our former views, in such a 
manner that these imperfections shall also be taken into consid- 
eraflon 
(To be concluded.) 
