96 THE NATURE AND [SECT. n. 



little, but not materially less, than that which would take place were it kept con- 

 stantly exposed to the sides. But to the convex surface the ends of the cylinder 

 have to be added. 



With the addition of the ends to the surface, the quantity of cooling in degrees 

 per second, (from art. 145.) becomes 



t'" = -07(24/+d) (T-Q 

 dv 



where / is the length of the cylinder in feet, d its diameter in inches, v the velocity 

 of the piston in feet per second, T the temperature of the steam less -jV part, and 

 t" the temperature of the air. The force is reduced to 



(t'" \ 

 1 Q=), the force on the piston, as in art. 148. 



157. When low pressure steam is employed, the temperature T will be 212 C > 

 and putting t" = 60, and supposing 



/ = , and v = 3-5, 

 12 



we shall have 



07 (9A r a- d) (T t'') 



= 15-2 degrees ; 



967 10U 



Therefore in low pressure engines there is a constant loss for all sized engines of 

 about TffTT of the power. When a casing is used and kept constantly filled with 

 steam, the loss of heat and constantly of power from the same fuel will be greater ; 

 because the surface will be constantly kept at the temperature of the steam. I hope 

 this will be sufficient to establish the truth, that the steam case is a useless addi- 

 tion to the expense of an engine. 



158. In a high pressure engine working at 300, the loss by a naked cylinder 

 is only about sV part of the force. 



159. The best mode of preventing loss is to put a case with an air-tight cavity 

 between it and the cylinder, instead of filling this case with steam ; and besides the 

 advantage of saving fuel the engine-rooms will not be heated so much. 



160. The single engine will lose more heat but not quite double the quantity 

 of the double engine ; hence we shall be about its amount in stating the cylinder 

 at losing A of the power. It will also lose double the quantity by the passage 

 of the steam from the boiler to the cylinder. 



161. In atmospheric engines the loss of force by cooling in the cylinder, when 

 a separate vessel is used for a condenser, is an interesting inquiry. Assuming that 

 the piston is kept steam-tight without the use of water, the loss must be greater 



