THE STEAM-ENGINE. 



433 



the pump barrel above it. As the air-pump piston approaches the top of its 

 barrel, the air and water above it will be drawn through the valve K into the 

 hot cistern B. The air will escape in bubbles through the water in that cis- 

 tern, and the warm water will be deposited in it. 



The magnitude of the opening in the condensing valve E, must be regulated 

 by the quantity of steam admitted to the cylinder. As much water ought to be 

 supplied through the injection valve as will be sufficient to condense the steam 

 contained in the cylinder, and also to reduce the temperature of the water itself, 

 when mixed with the steam, to a sufficiently low degree to prevent it from 

 producing vapor of a pressure which would injuriously affect the working of 

 the piston. It has been shown, that five and a half cubic inches of ice-cold 

 water mixed with one cubic inch of water in the state of steam would produce 

 six and a half cubic inches of water at the boiling temperature. If then the 

 cylinder contained one cubic inch of water in the state of steam, and only five 

 and a half cubic inches of water were admitted through the condensing jet, 

 supposing this water, when admitted, to be at the temperature of 32°, then the 

 consequence would be that six and a half cubic inches of water at the boiling 

 temperature would be produced in the condenser. Steam would immediately 

 arise from this, and at the same time the temperature of the remaining water 

 would be lowered by the amount of the latent heat taken up by the steam so 

 produced. This vapor would rise through the open exhausting valve I, would 

 fill the cylinder below the piston, and would impair the efficiency of the steam 

 above pressing it down. The result of the inquiries of Watt respecting the 

 pressure of steam at different temperatures, showed, that to give efficiency to 

 the steam acting upon the piston it would always be necessary to reduce the 

 temperature of the water in the condenser to 100°. 



Let us then see what quantity of water at the common temperature would be 

 necessary to produce these effects. 



If the latent heat of steam be taken at 1,000°, a cubic inch of water in the state 

 of steam may be considered for the purposes of this computation, as equivalent 

 to one cubic inch of water at 1,212°. Now the question is, how many cubic 

 inches of water at 60° must be mixed with this, in order that the mixture may 

 have the temperature of 100° ? This will be easily computed. As the cubic 

 inch of water at 1,212° is to be reduced to 100°, it must be deprived of 1,112° 

 of its temperature. On the other hand, as many inches of water at 60° as are 

 to be added, must be raised in the same mixture to the temperature of 100°, 

 and therefore each of these must receive 40° of temperature. The number of 

 cubic inches of water necessary to be added will therefore be determined by 

 rinding how often 40° are contained in 1,112°. If 1,112 be divided by 40, 

 the quotient will be 27 8. Hence it appears, that to reduce the water in the 

 condenser to the temperature of 100°, supposing the temperature of the water 

 injected to be 60°, it will be necessary to supply by the injection cock very 

 nearly twenty-eight times as much water as passes through the cylinder in the 

 state of steam ; and therefore if it be supposed that all the water evaporated 

 in the boiler passes through the cylinder, it follows that about twenty-eight 

 times as much water must be thrown into the condenser as is evaporated in 

 the boiler. 



From these circumstances it will be evident that the cold cistern in which 

 the condenser and air-pump are submerged, must be supplied with a consider- 

 able quantity of water. Independently of the quantity drawn from it by the 

 injection valve, as just explained, the water in the cistern itself must be kept 

 down to a temperature of about 60°. The interior of the condenser and air- 

 pump being maintained by the steam condensed in them at a temperature not 

 less than 100°; the outer surfaces of these vessels consequently impart heat 



VOL. II. — 38 



