CHAP, viz.] VARIOUS TYPES OF STEAM-ENGINES. 437 



varies, as we have said, with the engines. Watt's low-pressure 

 engines consume from 11 to 131b. of coal, Woolff's, about 61b ; 

 high - pressure engines, with expansion and condenser, consume 

 from 8 to lllb. per horse-power in an hour. These are the least 

 economical, but they counteract that defect by the advantages we 

 shall me'ntion presently. 



A word now with regard to the power of an engine in relation to 

 the dimensions of the cylinder and the A^elocity of the piston, or 

 which comes to the same thing, the nunrber of strokes of the piston 

 per minute or per hour, the pressure of the steam being known by 

 the reading of the manometer. 



How may the work done by the piston during its stroke in 

 the cylinder be calculated ? We will take an example which will 

 explain both the question and the reply that must be made to it, 

 Suppose there is a pressure of 4 atmospheres in a condensing engine, 

 or of 5 atmospheres in an engine without a condenser. The energy 

 exerted by the steam will in reality be the same in both cases, since 

 in the second the atmospheric pressure acts on the opposite face of 

 the piston to that on which the elastic force of the fluid acts. It is 

 found that for every square inch of the surface the work of the vSteani 

 will be about 151b., multiplied by 4, the number of atmospheres. 

 This must be multiplied again by the number of square inches the 

 surface of the piston contains. But this does not give the mechanical 

 work, which will be greater or less according to the length of the 

 cylinder or the excursion of the piston. To have the work in foot- 

 pounds, the result must be multiplied by that" length, so that we have 

 the following rule. 



Multiply the surface of the piston in square inches by its stroke 

 expressed in feet, by the effective pressure of the steam (that is, the 

 excess of the pressure one side over that on the other), and by fifteen, 

 and you have the number of foot-pounds which measure the work 

 done by the piston in each excursion. But the surface multiplied 

 by the length of the cylinder is the volume of the latter. 



Thus the work done is proportional to the pressure of the steam 

 and the volume of the cylinder. Suppose the diameter of the cylinder 

 is sixteen inches, and its length is fifteen inches. In this case the 

 engine is condensing, and let the vacuum of the condenser be assumed 

 perfect. Then steam pressure is 4 x 15 = 60 pounds per square inch 



