120 



OF THE GENERATION AND 



[SECT. in. 



to the excess of 50 Ibs. per square inch. In this case the whole surface of the 

 tube in contact with the water of the boiler could not exceed 52 feet ; and two- 

 thirds of this being taken as effective, we have 35 feet for the surface. 



The difference of the results in these trials is chiefly owing to a difference in 

 the density of the steam in the boiler, its state not having been ascertained ; and 

 though it might be done in an indirect manner from the number of strokes per 

 minute and the resistance, it would not be accurate enough to furnish us with 

 any useful conclusions. 



OF THE SPACE FOR STEAM AND WATER IN BOILERS. 



210. A boiler must obviously contain as much steam as will supply the 

 engine at each stroke without any material decrease in its elastic force ; and the 

 space will therefore depend on the manner the steam is to be supplied. If it be 

 admitted to the engine only during part of the time of the piston's descent, there 

 must be so much steam that the use of the quantity required may not lessen the 

 elastic force. If the steam be generated equably, and the space for it be only 

 equal to the quantity consumed at each stroke, and all the quantity be wanted 

 during the descent of the piston, the elastic force in the boiler will vary one-half, 

 and the loss of effect be very considerable: this subject is therefore worthy of 

 further inquiry, in order that we may see how far the maxims of practice are 

 confirmed by just principles. Without specifying the kind of engine, it is stated 

 that a boiler should have space for five or six times the volume of steam required 

 for a stroke ; l others mention eight times : Dr. Young quotes a remark that it 

 should contain ten times the volume, 2 and Prony has stated that it is one of the 

 advantages of a double acting engine, that it requires a smaller boiler than a single 

 acting one. * 



211. Let it be supposed that the action of the fire is uniformly the same, and 

 that during a unit of time it generates a unit of volume of steam, and that this 

 volume is sufficient to supply the engine ; but jhat the whole of it is required in 



1 Millington's Epitome of Natural Phil. p. 251. 

 3 Architecture Hydraulique, vol. ii. p. 106. 



- Natural Phil. vol. ii. p. 259. 



