02 
THE NATURE AND 
[sect, IX. 
which will enable us to compare with practice after considering the other causes 
of loss. 
145. Loss of force by cooling. But much of the force of the steam will also 
he lost in the passage through the steam pipe by cooling. The quantity of steam 
exposed during a second is as the area and velocity of the steam ; or 
= cubic feet; 
144 
a 
being in inches, the rest in feet. 
The surface is as the length and circumference, or 
4 la 
127 
square feet. 
Hence the loss of heat being directly as the surface, and inversely as the quantity 
exposed, we have for cooling in metals 
2-1 (T - t") v 41 a v 144 _ 
60 X 12 d a v ~ 
the loss of heat in a second ; or rather the loss of heat which the quantity passing 
in one second experiences. 1 By reducing the expression to its lowest fraction it 
becomes 
1-71 (T - t") 
d v 
In this equation 
T is the temperature of the surface of the steam pipe, which will be about 
one-twentieth less than that of the steam; 
t" is the temperature of the air, 
l the length of the pipe in feet, 
d its diameter in inches, 
and v the velocity in feet per second. 
146. In applying this formula to find the loss of heat, there are no other 
circumstances to be considered ; but in its application to determine the loss of 
elastic force, there is a most important point, to which I would particularly direct 
the attention of manufacturers of engines. It is the degree to which the 
temperature of the steam is reduced by passing through the pipe. It is said to be 
frequently as much as would reduce its temperature below 212°; when this is the 
case, we know that part of the steam must become water, and the rest of it become 
of the force equivalent to a temperature of 212°, and therefore all the excess of 
force which was generated in the boiler, would be destroyed by the cooling in the 
passage to the engine. 
1 Tredgold on Warming and Ventilating, art. 44.