SECT. ,.] CONDENSATION OF STEAM. 121 



some less time i ; and that c is the capacity of the space for steam in the boiler, and 

 p the elastic force at the commencement of letting on the steam. Then c + t 1 

 is the quantity of steam in the space c at the end of letting on the steam ; and the 

 elastic force being inversely as the space, it will be 



p(c + t-\) 

 c 



at the time it is shut off ; and the variation will be 



Now in a single acting engine the time t when it acts at full pressure is one- 

 half, hence -j is the loss of elastic force ; but if we make c = eight times the 

 quantity required, the loss is only TV p, or the elastic force varies only T V, or about 

 one pound on the square inch. 



212. If the steam be cut off before the stroke be completed, the variation will 

 obviously be greater ; for example, in a single engine cut the steam off at half the 



o 



descent, and the variation of elastic force in the boiler will be p -j-, or TT nearly, 



when the capacity for steam in the boiler is eight times the quantity required 

 for the stroke. 



213. In the double acting engine, the steam acting at full pressure, the time 

 t is nearly the same as the time denoted by 1, and about three times the quantity 

 required for the stroke may be sufficient ; but if the steam be cut off at any 

 fractional portion of the stroke, put t equal to that fraction, and it will be found 

 to what the capacity must be increased, to render the variation of force incon- 

 siderable. Thus if it be cut off at half the stroke, then 



, (1 - f) = P 



p \~~rJ 27' 



the same as in single engines, and we should not make c less than 8. But it must 

 be remarked, that it is in all these cases c times the volume of steam as it is in 

 the boiler, and not c times the capacity of the cylinder, because during the time 

 the steam acts by expansion, there is none entering the cylinder. 



214. For each cubic foot of water converted into steam in an hour by a low 

 pressure boiler, we may assume that one cubic foot of steam is used at a stroke 

 without material error ; and if, as agrees with other parts of the arrangement of 

 an engine, the variation be limited to ^V of the force of the steam, we shall have 



'-^r- -55 r 30 (1 - = c. 

 That is, calling the interval 1 from the time the steam valves are opened to the 



