438 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



above the atmospheric pressure, and the vacuum is 151b. per square 

 inch below it. Thus the excess of pressure is 751bs. ; the work in 

 one excursion of the piston will be TT. 8 2 x 75 x 1^ foot-pounds, or 

 about 18,800 foot pounds. The whole to-and-fro motion of the piston 

 then would do 37,600 foot-pounds of work. 



This gives the work of the engine for one to-and-fro motidn of the 

 piston, so that we must know besides the number of these motions 

 which take place in a minute or hour to find definitely in horse-powers 

 the power of the engine. 



This velocity of the piston is very variable, but it seldom exceeds 

 sixty strokes a minute, or a stroke' a second. If it works with its 

 maximum velochv^, the power of the engine would be 37,600 foot 

 pounds per second, or about sixty- eight horse-power. 



V. HISTORICAL SKETCH OF THE STEAM-ENGINE. 



The first steam-engines actually employed in practice were those 

 of Savery (1696 1698). Their principle had been given by Papin, 

 since, as Arago says, " Papin was the first who attempted to combine 

 in one heat engine the elastic force of steam with the property that 

 steam possesses, and which he pointed out, of condensing with cold." 

 The design of Savery's lifting engine, reproduced in Fig. 305, as far 

 as its essential arrangements are concerned, shows that the steam 

 was produced in a separate vessel B (the boiler). The steam 

 first filled the vessel s and the pipe A, out of which it drove the air ; 

 then closing the tap c, and opening the tap e, leading from a reservoir 

 full of cold water, he produced condensation of the steam in the 

 vessel S; a vacuum was formed, and the water of the reservoir R rose 

 and partly filled the vessel and the pipe. A jet of steam coming then 

 from the boiler and pressing on the surface of the liquid forced it to 

 rise to a height depending on the pressure. A fresh condensation 

 then took place, a fresh. action of the steam, and so on indefinitely. 



"To raise the water to the height of only 200. feet, for example, 

 Savery was forced," says Arago, " to bring the steam of his boiler to 

 a pressure of .six atmospheres ; hence there were continual disarrange- 

 ments of the joints and melting of the solders as well as dangerous 

 explosions ; so that in spite of the title of his work, The Miner's 

 Friend, his engines were of no use to the mines. They were only 



