148 OF THE GENERATION AND [SECT. HI. 



\ 



quantity of water necessary for condensation. And since a cubic inch of water 

 produces about a cubic foot of steam of the rarity it is in the cylinder of an engine 

 working at this temperature, and one-tenth being added for each foot of the 

 capacity of the stroke, 23i x Tl = 25f cubic inches for each foot of the contents 

 of the stroke of the cylinder. 1 



If x = 130, it requires of cold water only 14 times the weight of the steam to 

 condense it, and for 120 it requires 16'2 times the weight. 2 



The force of steam at 100 is 2-08 inches of mercury, its force at 130 is 4-81 

 inches ; consequently the gain of power is 2*73 inches, or about one in thirteen, by 

 condensing at the lower temperature. 



If the temperature of the cold water be 70, and of the condenser 130, then we 

 find cold water 18 times the weight of the steam will condense it ; and that it 

 requires 37 times the weight to condense at 100, when the cold water is at 70. 



285. From these equations the comparative effects of different temperatures 

 may be calculated, and the economy of using or sparing water will be known and 

 acted upon, instead of the usual method of endeavouring to get the greatest power 

 of the steam in places where water is expensive. 



When steam is of considerable density, it does not condense freely ; the reason is 

 obvious, the same surface of injection water acting on steam of greater density, and 

 consequently containing a greater proportion of heat, it abstracts the heat more 

 slowly. To avoid this, the condenser should be so large that the steam may 

 expand to the bulk corresponding to a pressure not greater than about one atmo- 

 sphere and a half. But it is better to make the steam act expansively in the 

 cylinder by Watt's method, (art. 27.) or expand in a second cylinder by Horn- 

 blower's method, (art. 32.) 



When a lower temperature than 180 cannot be obtained by condensation, it is 

 not worth the extra expense, and at 180 we have for low pressure steam 



w _ (1000 + 220 - 180) _ ,, . nearl . 

 180 ^ o^ 



or eight times the quantity of water required for steam will be necessary to 

 condense it. 



286. These computations apply to where condensation is made in a separate 

 vessel, the first idea of which we owe to Mr. Watt. When the condensation is 

 made within the cylinder, the metal of the cylinder has to be cooled down to the 

 temperature of condensation as well as the steam, and a large proportion of the 



1 Mr. Watt says a wine pint, or 28| inches, is " amply sufficient." Robison's Mech. Phil, 

 vol. ii. p. 147. 



2 The usual temperature is about 120, or just what the hand can bear. 



