92 THE NATURE AND [SECT. n. 



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 

 be 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 



*-rf 4/a 

 = I x -r-r = , square ieet. 



I "- I - a 



Hence the loss of heat being directly as the surface, and inversely as the quantity 

 exposed, we have for cooling in metals 



8-1 (T-Q 4/a 144 



60 12rf av~ 



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 



f , _l-7 I (T - f) 

 dv 



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' 1 is the temperature of the air, 

 / the length of the pipe in feet, 

 d its diameter in inches, 

 and t; 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. 



