CHAP. X.] 



HOT-AIR ENGINES. 



507 



the metallic sheets where it was heated at the expense of the heat 

 given up by the hot air previously driven out." 



Ericson subsequently modified his original form of steam-engine. 

 He suppressed the metal sheets, and then the hot-air, after having 

 worked upon the driving piston, is directly rejected from the engine. 

 So that it is a single acting engine, and to start it the fly-wheel must 

 first be moved by the hand. Laubereau's engine, that we are about 

 to describe, is in this last respect similar to Ericson's, and will enable 

 us to understand its machinery. It is, too, as simple as possible. 



The driving machinery in Laubereau's engine is composed of two 

 metal cylinders, A, B, of unequal diameter, whose interiors communi- 

 cate together by a tube t. In the first, which is open at the top, a full 

 sized piston, p, moves, which fills 

 the cylinder hermetically and pre- 

 vents any communication between 

 the inside of the cylinder and the- 

 outer air. This is the driving 

 cylinder and piston of the engine. 



The large cylinder B is com- 

 pletely closed at its upper and 

 lower ends, both of which are 

 concave exteriorly. A thick pis- 

 ton, P, formed of a badly conduct- 

 ing substance, such as plaster, also 

 moves in the cylinder but without 

 touching its sides-. A doubly con- 

 cave form is given to it in order that it may fit either end of the 

 cylinder. This we may call the feeding cylinder, because it is the 

 air that is contained in it, that by being alternately heated and cooled, 

 works on the driving piston, or, on the contrary, stops that action 

 at each period of the movement. In order to obtain these successive 

 effects, the source of heat (in this case a jet of gas) warms the exterior 

 concave surface of the lower side of the cylinder, and consequently 

 the air beneath the piston P. The pressure of this air then exceeds 

 in the larger cylinder the atmospheric pressure, and hence thrusting 

 the piston p from below gives it an ascending motion which is com- 

 municated by the usual appliances to the shaft and fly-wheel of the 

 engine. The piston P then descends again, and fits on to the lower 



FIG. 331. Section of the cylinders in Laubereau's 

 engine. 



