SECT. II.] 
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 f that after the heat 
has been lost, 
f-f 
f 
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, /' 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 
T = 220 - — = 209° 
20 
1*71 (T—f") _ 1-7 x 1? - /onQ - m ' 
d v 
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 tht 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.)