394 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



is in every other respect a step backwards from Papin's. Iii fact, the 

 elastic force of the steam was employed in it to drive back the water 

 directly, while Papin used it to produce motion in the piston a 

 movement which it is only necessary to transform by purely mechani- 

 cal processes to render the engine a universal prime mover. 



We will briefly describe the principle of the modern steam-engine, 

 and the principal parts of which it is composed. 



First, and above all, a means must be devised to develop the force, 

 that is to say, to produce and collect a certain quantity of steam. 

 This is accomplished by heating a boiler filled, or partly filled, with 

 water. This is the steam generator, one of the three essential parts or 

 constituents of the engine. 



From the boiler the steam passes into a chamber of cylindrical 

 form, divided into two parts by a movable piston ; it is here that, by 

 special arrangements, the steam acts first on one side and then 011 the 

 other of the piston, so as to give it an alternate to and fro motion, 

 which is the direct object of the machine. 



This form is called a double-acting engine. All the earlier and 

 many even of the most efficient engines of the present day, the 

 " Cornish engines," for example, are single-acting ; that is to say, the 

 steam is employed only to drive the piston one way, it is then allowed 

 to escape into another vessel purposely kept cool, where it condenses, 

 leaving the unbalanced pressure of the atmosphere to drive the 

 piston back again. 



The cylinder, the piston, and accessories, which distribute the 

 steam in the two chambers of the cylinder, constitute that part of the 

 engine called the prime mover. It is the engine, properly so-called, 

 the action of which would not be well understood without entering 

 into further details. 



Consider Fig. 273, which represents the steam-engine reduced to 

 its essential parts, c is the boiler where the water is converted into 

 steam, which fills its upper part as well as the pipe vv. This pipe 

 conducts the elastic vapour into a chamber b next to the cylinder, 

 called the valve chest. Two taps R'K' admit the steam, according 

 as one or the other is open into the upper chamber B or the lower 

 chamber A of the cylinder. First suppose the upper tap open and 

 the lower closed. The steam passes into B, where it presses upon 

 the piston, and tends to impress upon it a descending motion in 



