436 . THE APPLICATIONS OF PHYSICAL FORCKS. [BOOK -iv. 



at which is expressed as 33,000 lb. raised 1 foot in a minute. But 

 we must not make a mistake. The work of the steam is supposed 

 continuous, and the engines work night and day without resting. An 

 engine of one horse-power does in a day. of twenty-four hours 1440 

 times this work, but a real living horse, on the contrary, requires to 

 rest, and if he works for eight hours a day lie will not do more than 

 one third of the work of the engine. 



In reality this value is still too high. Watt's figures, if we may 

 judge by more recent experiments, were applicable to horses of more 

 than ordinary power, and these were probably overdriven. It follows 

 from the experiments to which we have alluded that a horse of ordinary 

 strength, walking for eight hours turning a windlass, would do only 

 17,820 foot-pounds per .minute. 



We see then, on a comparison of the two sets of figures relative 

 to the work of the engine and that of the animal, that in reality, to 

 replace an engine of one horse -power, in order to turn the .same 

 windlass without ceasing, a little more than five horses and a half 

 must be employed. 



What constitutes the power of a boiler is the quantity or the 

 weight of steam that it is capable of producing in an hour when 

 in full work. Now it is chiefly on the heating surface that this 

 quantity depends, so that other things being the same, the 

 generator that offers to the fire and the gases of combustion the 

 largest amount of heating surface is the most powerful. 



With regard to the consumption of coal, it is evidently in relation 

 to the heating surface, but it varies from one engine to another, 

 according to the type of the engine, whether it works at high, low, or 

 medium pressure, and whether it works with or without a con- 

 denser, with or without expansion. Experiment shows the following 

 facts. 



It is found by practice that for each horse-power, the heating 

 surface varies from 10 to 15 square feet. A steam engine of 10 

 horse-power must have a generator with a heating surface of from 

 100 to 150 square feet. The quantity of steam produced in an hour 

 is, on an average, 441b. per horse-power, so that the boiler of an 

 engine of 10 horse-power should be able to convert into steam 4401b. 

 or about 44 gallons of water per hour. 



As to the consumption of coal per hour and horse-power, it 



