THE STEAM-ENGINE. 521 



the useful effect produced by a given weight of fuel, the greater will be the duty, 

 however long the time may be which the fuel may take to produce the useful 

 effect. 



The proportion of the stroke to the diameter of the cylinder must be de- <; 

 termiried by the velocity intended to be given to the piston. With the 

 same capacity of cylinder, and the same evaporation in the boiler, the 

 velocity of the piston will augment as the magnitude of its diameter is dimin- 

 ished. 



The proportion of the diameter to the stroke of the cylinder is very various. 

 In engines used for steam-vessels the length of the cylinder very little exceeds \ 

 its diameter. In land-engines, however, the proportion of the length to the di- 

 ameter is greater. It is maintained by some that the proportion of the diame- 

 ter and length of the cylinder should be such as to render its surface exposed 

 to the cooling of the external air, the smallest possible. Tredgold has main- 

 tained that since, during the stroke, the steam is gradually exposed to contact 

 with the surface of the cylinder from the top to the bottom, the mean surface 

 exposed in contact with steam being half that of the entire cylinder, the pro- 

 portion of the diameter to the stroke should be such that the surface of half the 

 length of the cylinder, added to the magnitude of the top and bottom, shall be 

 a minimum. If this principle be admitted, then the best proportion of the di- 

 ameter to the stroke would be that of one to two, the length of the stroke being 

 twice the diameter of the cylinder ; but since the whole surface of the cylinder 

 is constantly exposed to the cooling effects of the air, and since in the inter- 

 vals of the stroke there is no sensible change of the temperature of the surface, 

 the loss of heat by cooling will in effect be the same, especially in double- 

 acting engines, as if the cylinder were constantly filled with steam. If this be 

 admitted, then the object should be to give the cylinder such a proportion, that 

 its entire surface, including the top and bottom, shall be a minimum. The 

 proportion given by this condition would be very nearly that which is observed 

 in the cylinders of marine-engines, viz., that the length of the cylinder should 

 be equal to its diameter. 



If in a low-pressure engine the pressure of steam in the cylinder be taken at 

 17 Ibs. per square inch, then the volume of steam will be about fifteen hundred 

 times that of the water which produces it. For every cubic foot of water, 

 therefore, in the effective evaporation of the boiler, 1,500 cubic feet of steam 

 will be passed through the cylinder. If it be intended that the motion of the 

 piston shall be at the rate of 25 strokes per minute, or 1,500 strokes per hour, 

 then the capacity of that portion of the cylinder between the steam-valve and 

 the piston at the end of the stroke, must consist of half as many cubic feet as 

 there are cubic feet per hour evaporated in the boiler. If the steam, therefore, 

 be cut off at half stroke, the number of cubic feet of space in the cylinder will 

 be equal to the number of cubic feet of water effectively evaporated by the 

 boiler; and if a cubic foot of water effectively evaporated be taken as the 

 measure of a horse-power, then there would be as many cubic feet, in 

 the capacity of the cylinder as is equal to the nominal power of the en- 

 gine. 



The duty of engines varies according to their form and magnitude, the cir- 

 cumstances under which they are worked, and the purposes to which they are 

 applied. In double-acting engines working without expansion, the coal con- 

 < sinned per nominal horse-power per hour varies from 7 to 12 Ibs. An exami- 

 ) nation of the steam-logs of several government steamers made by me a few 

 I years since, gave, as the average of consumption of fuel at that time of the best 

 ) class of marine-engines, about 8 Ibs. per nominal horse-power per hour. Since, 

 ( however, no account could be obtained of the actual evaporation of water in the 



