CHAP, x.] GAS-ENGINES. 509 



machine its constant action ; a pump, worked by the driving-shaft 

 constantly brings into the double wall of the large cylinder the cold 

 water that is necessary for the cooling of the air that has done its 

 work, and takes it away when heated by the heat which the air parts 

 with to the sides of the cylinder. 



The hot-air engines, &c., as well as all gas-heat engines, as distinct 

 from steam-engines properly so-called, are useful for small operations, 

 which require but comparatively slight power, which can be often 

 interrupted. The ease with which they are set going makes them 

 economical in this respect ; but they would not be so for providing a 

 continuously acting force of large amount, as in great manufactories. 



Hot air has certainly one advantage over steam namely, that be- 

 tween wide limits of temperature the pressures are much smaller, so 

 that the quantity of heat consumed, and consequently work done, 

 may be very great without there being any fear of the covering of 

 the cylinders being deficient in resisting power ; but also, 

 practically, if a large motive force is required, the surface of the 

 pistons must be greatly enlarged. On the other hand, at high tem- 

 perature, the hot air burns the fittings of the pistons, and oxidizes 

 and spoils the metallic surfaces with which it comes into contact. 

 Steam has none of these disadvantages. 



III. GAS-ENGINES. 



We now come to some other prime movers which are beginning 

 to be employed pretty frequently in small operations we refer to 

 gas-engines. 



It is still the expansion of air that supplies the motive force to these 

 machines ; but instead of being expanded by the action of a source 

 of heat maintained beneath the chamber containing it, it is by the 

 effect of the disengagement of heat produced by the explosion of 

 an explosive mixture. This mixture is formed of air and illuminating 

 gas in suitable proportions. 



The different methods of producing this explosion have given 

 rise to various arrangements of the gas-engines. In Lenoir's engine, 

 the explosive mixture formed of twenty parts of air to ten parts of gas 

 is kindled by the successive sparks of an induction coil. In Hugon's 



