506 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



incrustations of the boilers after the liquid has been a long time at 

 work ; and that the ammonia preserves also the sides from oxidation. 

 But the chief disadvantage is this, as for all combined steam-engines, 

 the difficulty of preventing the escape of the gas ; and the danger 

 arising from a mixture with the air of a substance whose action on 

 the respiratory organs is so dangerous. 



II. HOT-AIR ENGINES. 



In the engines we have just described, the motive power employed 

 is that of the combination of steam with the vapour of a more 

 volatile liquid, or with a gas that is forced by the heat to disengage 

 itself from the solution. In them steam still therefore always plays 

 an important part. An attempt has been made to substitute for it 

 an entirely distinct elastic force, namely what we may obtain by 

 heating a permanent gas such as air, or by setting fire to an explosive 

 gaseous mixture, whence arise two new kinds of prime movers, hot- 

 air engines and gas-engines. They are, however, still heat engines, for 

 it is still from the heat employed that the mechanical work 

 obtained. 



The first attempts at employing heated air as a motive force date, 

 it appears, from Montgolfier. One of the inventors also of photography, 

 J. Mepce, occupied himself with the same problem. But in 1816, 

 Eobert Stirling constructed a hot-air engine, which, according to a 

 competent authority on these subjects, is at the same time the simplest 

 in theory and the most approved by experience. 



Later, a Swedish engineer, Ericson, planned and constructed a 

 hot-air engine which worked on board an American ship in 1853. 

 M. Collignon defines the principle of this new prime mover in these 

 terms : 



" In his first engine Captain Ericson placed a regenerator formed 

 of a great number of metallic plates, in the path of the heated air as 

 it left the prime cylinder when the piston was making its retrograde 

 movement. The cylinder was heated directly by the fire, and it 

 transmitted the motion obtained in it to a feeding cylinder, which was 

 a true pump taking the air from the atmosphere and compressing it in 

 a reservoir, whence the air reached the moving piston after traversing 



