SECT. v.J NONCONDENSING ENGINES. 177 



foot of water forms at the temperature or force in the boiler, (art. 121. or tables 

 at the end,) the result will be the cubic feet of water consumed per minute ; 

 and the quantity of water, and consequently the quantity of fuel, (art. 190.) 

 will be known ; but the supply of water should be a little in excess. 



370. The purposes to which noncondensing engines of this kind have been 

 applied, are to impelling steam carriages, moving materials within deep mines, 

 draining mines in places difficult of access, driving machinery in places where 

 water cannot be obtained at a moderate expense, and in various instances where 

 low pressure steam was equally available ; but for most of these purposes it is 

 inferior to the next species : the sole advantage it possesses being that of uniformity 

 of force in every part of the stroke ; which in some instances is desirable, but in 

 others hurtful. 



NONCONDENSING ENGINES TO WORK BY EXPANSION. 



371. Second Species. The only difference required in the construction of a 

 noncondensing engine to enable us to use the expansive power of the steam, is in 

 the arrangement for opening and closing the steam passages. The steam must be 

 admitted from the boiler only during a part of the stroke, and then shut off, but 

 the passage for the escape of the steam should be open during the whole of the 

 stroke. When the passage from the boiler is shut, the steam acts by expansion, 

 and the power it affords by expansion is wholly in addition to that which is ob- 

 tained by the preceding species ; whence the economy of this method. 



372. The most important question is to determine that point in the length 

 of the stroke at which the steam should be cut off, so that it may afford the 

 greatest quantity of useful effect from a given quantity of steam ; for then a given 

 quantity of fuel produces the greatest useful effect. Now we have shown the 

 resistance from friction, &c. to be nearly, if not exactly, 0*4 of the whole force of 

 the steam in the boiler, (art. 367.) and it is obvious, that when the steam has 

 expanded till its excess of force be equal to this resistance, it will produce no 

 further useful effect ; and also that as far as the expansion exceeds this limit, there 

 must be a decided loss of power. Hence, if we consider the capacity of the cylinder 

 to be 1, the force being inversely as the space the steam occupies, it must be, as 

 the whole force of the steam in the boiler is to 1, so is the whole force on the 

 piston, when it is just equal to the friction, to the portion of the stroke when the 

 steam should be cut off. 1 That is, if the whole force in the boiler be 120 inches 



1 Put/ = the force iu the boiler in inches of mercury, and = the portion of the stroke made 

 before the steam is cut off, then, 



