MOTIVE POWER OF HEAT. 117 



bustible, are considered the best. They consist of 

 a small cylinder, which at each pulsation is filled 

 more or less .(often entirely) with steam, and of a 

 second cylinder having usually a capacity quadruple 

 that of the first, and which receives no steam ex- 

 cept that which has already operated in the first 

 cylinder. Thus the steam when it ceases to act 

 has at least quadrupled in volume. From the 

 second cylinder it is carried directly into the con- 

 denser, but it is conceivable that it might be carried 

 into a third cylinder quadruple the second, and in 

 which its volume would have become sixteen times 

 the original volume. The principal obstacle to the 

 use of a third cylinder of this sort is the capacity 

 which it would be necessary to give it, and the large 

 dimensions which the openings for the passage of 

 the steam must have. We will say no more on this 

 subject, as we do not propose here to enter into the 

 details of construction of steam-engines. These 

 details call for a work devoted specially to them, 

 and which does not yet exist, at least in France.* 



* We find in the work called De la Eichesse Minerals, by 

 M. Heron de Villefosse, vol. iii. p. 50 and following, a 

 good description of the steam-engines actually in use in 

 mining. In England the steam-engine has been very fully 

 discussed in the Encyclopedia Britannica. Some of the 

 data here employed are drawn from the latter work. 



