SECT, vi.] CONDENSING ENGINES. 187 



passage E for the water used for injection to run out at, provided with a self- 

 acting valve F to prevent it flowing back ; a valve G for the air contained in the 

 water to escape at is also necessary. Mechanism for opening and closing the 

 valves is connected to the engine beam ; and a small supply of water by a pipe 

 is constantly furnished to the top of the piston, to keep the packing saturated, so 

 as to be steam-tight. 



394. The operation is simple : the beam is so balanced that the steam being 

 admitted by the steam valve V below the piston, it rises to the top of the stroke ; 

 the steam valve is then shut, and the injection cock D is opened, and a jet of 

 cold water rises through I, which condenses the steam to a lower degree of 

 elasticity, and the water runs out by the passage E at the valve F ; the pressure 

 of the atmosphere on the piston P being unopposed, it forces it down, and the air 

 extricated by the water is expelled towards the termination of the stroke at the 

 valve G. 



The steam and injection cocks are moved by tappets on a bar moving vertically, 

 and connected to the beam. The steam valve should close and the injection cock 

 open just when the up stroke is completed, and the period of closing the injection 

 cock should be adjusted to the power the engine is to exert ; the steam valve 

 ought to open with the rise of the piston. 



395. The proportions of the parts. The length oi the cylinder should be 

 twice the diameter, art. 329. The velocity in feet per minute should be ninety- 

 eight 1 times the square root of the length of the stroke in feet, (art. 342.) the engine 

 being supposed to be applied to raise water. The area of the steam passages will 

 be found by this proportion, as 4800 is to the velocity in feet per minute, so is 

 the area of the cylinder to the area of the steam passage, art. 154. The tempe- 

 rature for condensation which affords the greatest useful effect will be found by 

 art. 166. If the area of the cylinder in feet be multiplied by half the velocity in 

 feet, and that product by T23 added to 1*4 divided by the diameter in feet, (art. 

 163.) the result divided by 1480 will give the cubic feet of water required for 

 steam per minute. If from 1220 the temperature of condensation be deducted, 

 and the result divided by the difference between the temperature of the cold water 

 and the temperature of condensation, the quotient will be the number of times the 

 quantity of water required for injection must be greater than that required for 

 steam (art. 284.) ; in general it will be about twelve times the quantity, but it had 

 better be a little in defect than excess. The aperture for the injection must be 

 such that the above quantity of water will be injected during the time of the 

 stroke. The moving force in the first instant is only that due to the height of 



1 See note p. 190. 



