8ECT . .] PROPERTIES OF STEAM. 93 



147. A knowledge of this cause of the reduction of the force of steam to 

 atmospheric elastic force, and of the importance of not losing force where either 

 economy of heat or of space is desirable, creates a strong desire to know its amount, 

 knowing that the most esteemed manufacturers of steam-boat engines cause the 

 steam to pass round between the jacket and the cylinder; as if to expose it 

 as much as possible to the cooling 'effect of the atmosphere, to reduce its elastic 

 force before it enters the cylinder to exert its power. 



148. The reduction of the temperature of steam reduces its elastic force to 

 that of a lower temperature, and during this reduction a portion of the steam 

 becomes water. If/ denote the elastic force in the boiler, and /' that after the heat 

 has been lost, 



will be the quantity reduced to water, and this multiplied by its heat of conversion 

 into steam must be equal to the heat the whole has lost by cooling ; therefore 



And here it will be remarked, that when t'" is equal to the whole heat of con- 

 version, f will be nothing ; or the whole will be cooled into water as it is in an 

 apparatus for warming buildings. We are now in a condition to give an answer 

 to the question of what is the loss of force in any particular case. Let the tempe- 

 rature of the steam be 220, and its force 35 inches of mercury, the length of the 

 steam pipe 12 feet, its diameter 6 inches, the velocity of the steam in the pipe 

 80 feet per second, and the temperature of the air 60. Then by art. 145. 

 we have 



= 220-- =209 

 20 



-O = 1-7 x 12 x (209-60) = 



dv 6 x 80 



and therefore by the equation above we have 



'('-) -(>-) -"" 



consequently there is in this case a loss of force equivalent to 0*23 inches of 

 mercury, or TTTT of the force ; but this is one of the most favourable of the cases 

 that usually occur in practice. In steam-boat engines where the steam has to 

 pass round the cylinder, the force in the cylinder is stated, from observation, 



1 The number 967 is here taken as the heat of conversion into steam, but in general I use 

 1000 as more accurate. (See art. 82.) 



