Mechanical Theory of Heat to the Steam Engine. 203 



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. 



In the above-mentioned machines in which, besides water, a 

 second more volatile substance is applied, the interval (2\ — T Q ) 

 is made larger because T 0 is made lower. The idea has also sug- 

 gested itself in the same manner to increase the interval on the 

 upper side by adding a third fluid less volatile than water. The 

 fire would then immediately evaporate the least volatile of the 

 three substances ; this, by its condensation, the second, and this 

 the third. According to the principle it is not to be doubted 

 that this combination would be advantageous, how great how- 

 ever, the practical difficulties will be which are opposed to the 

 execution, cannot a priori be determined. 



26. 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. 



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 3 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- 

 eration. 



(To be concluded) 



