174 OF THE CONSTRUCTION OF [SECT. v. 



properly managed, the elasticity of the steam destroys the momentum of the piston, 

 and recoils it back without loss of force. 



This will afford the reader a general notion of the action of steam in non- 

 condensing engines, and prepare him for entering more closely into their minutiae. 

 I have divided them into two kinds ; and of the varieties depending on different 

 forms of construction there is an immense number. 



360. Noncondensing engines, 



fLeupoId, (art. 12.) 1720. 



t . f e . j Watt, (art. 26.) 1769. 



(1. the eenerative force of steam, < _ 



] Trevithick, (art. 56.) 1802. 



Acting by -I (^Evans, (art. 58.) 



r Oliver Evans, (art. 58.) 

 I 2. the generative and expansive force of steam, < 



I Taylor and Martineau. 



361. First Species. When the power is derived solely from generating the 

 steam under pressure, (art. 295.) the construction of the parts constituting the 

 engine is very simple. The common method is represented in Fig. 1. Plate iv. 

 With the object of losing as little heat as possible by the cooling of the cylinder, it 

 is generally placed partly within the boiler, and the steam is admitted and let out 

 by a four-passage cock A, placed just without the boiler, with a throttle valve V to 

 regulate the entrance of the steam. The steam escapes to the atmosphere by a 

 pipe E, which is generally surrounded by water W, for the supply of the boiler, 

 which has the effect of partially condensing the escaping steam, and facilitating its 

 escape from the cylinder, as well as of increasing the temperature of the water 

 before it be admitted to the boiler. 



362. This construction is defective, in as far as there must be an absolute loss 

 of all the steam in those parts of the admission pipes which are between the cock 

 and the cylinder ; and the great density of high pressure steam renders the loss of 

 power considerable. To avoid it, there should be two double-way cocks, one at the 

 bottom and one at the top of the cylinder ; or the passages may be opened and 

 closed by a slide, as shown in Fig. 2. where it will be obvious that the spaces 

 between the stops and the cylinder are as small as possible. 



363. If we now trace the action of the steam, and the opening of the passages, 

 we shall find to what points to attend in perfecting the operation of the engine. 

 In Plate iv. Fig. 1. represents an engine, of which C is the cylinder, and P the 

 piston at the top ready for descending. The motion of the cock A might end with 

 the end of the stroke, but the steam would be cut off", and indeed all the passages 

 stopped when it is half turned. The closing, when quickly done, commences 

 sufficiently before the end of the stroke, to effect the recoil of the piston (art. 359.) ; 



