430 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



through the cylinder and which the piston completely surrounds. 

 This arrangement diminishes the surface of the piston exposed to 

 the action of the steam, and this diminution must be compensated 

 by increasing the diameter of the cylinder. The disadvantage of this 

 very simple arrangement is easy to understand : for one thing, the 

 steam is more quickly chilled, because the surface exposed is more 

 considerable ; and for another, leakages are more easily produced 

 both round the collar and the grooves in which the piston moves. 



This kind of transmission is principally employed in English 

 steamships. 



A French manufacturer, M. Carre, invented and constructed the 

 first oscillating engine, in which there was no connecting-rod, the 

 piston-rod itself being jointed directly to the crank of the shaft. 



The cylinder of oscillating engines is supported by two trunnions, 

 like a piece of artillery on its carriage. The trunnions are 

 hollow, and serve, one for the steam port and the other for the 

 exhaust port. In other respects the distribution is regulated by a 

 slide-valve as in ordinary engines. A distinction is drawn between 

 horizontal and vertical oscillating engines, according to the mean 

 direction of the cylinders in their successive oscillations. 



III. EOTATORY STEAM-ENGINES. 



It still remains for us, while studying the various types of steam- 

 engines, to speak of a kind of engine which differs from all the 

 others that we have passed in review up to this point in the very 

 principle of its machinery. We mean the rotatory engines, so called 

 because the part oh which the steam acts directly, or which corre- 

 sponds to the piston in the cylinder engines, receives a motion which 

 is directly circular and continuous. The problem of the transforma- 

 tion of the motion is not involved in these engines. 



The idea of solving the question of motion by steam in this way 

 is not new. It occurred to Watt in 1782 ; but the disadvantages of 

 this arrangement have not encouraged large manufacturers to assist 

 in improving the tentative essays in this direction: even now, in 

 spite of the improvements introduced in the construction of rotatory 

 engines, it is only in very special cases that they can be made use of 

 in practice. 



