SECT, ii.] PROPERTIES OF STEAM. 97 



than in the single steam engines by the amount lost in cooling the inside of the 

 cylinder half the time ; hence the value of / the length of the cylinder must be 

 increased one-half, besides doubling the area exposed in a given time. This will 

 render the equation for the loss of temperature, (art. 156.) 



?' = '14 (36 I + <*) (T - t") 



' dv 



With the proportions and temperatures of the example, (art. 157.) the loss by 

 cooling is about A of the power; therefore it is not this species of loss which 

 should prevent this simple kind of engine being employed for mines. 



If water be applied to keep the cylinder tight, the additional loss from convert- 

 ing this water into vapour will be considerable. If the mean temperature of this 

 water be 180, the effect of each foot of area will be to abstract, or to destroy a 

 cubic foot of steam per minute, this being the quantity of evaporation from a foot 

 of surface of water sustained at that temperature. Therefore in an engine working 

 at the rate of 170 feet per minute, that is, expending 85 cubic feet of steam of 

 atmospheric density per minute, for each foot in area of the cylinder the loss will 

 be -$ = A of its power ; hence, adding this to the cooling effect, we have iV + iV 

 = about TV of the power. 



162. In the common atmospheric engine where the injection is made within 

 the cylinder, the only person who had attempted to calculate the loss of force was 

 Smeaton ; of which some account has been given by Mr. Farey, in Rees's ' Cyclo- 

 paedia.' The mode of calculation is not very clearly given, and it was formed at 

 a time when the properties of heat were less known. 



163. Cylinders are usually made of the same thickness, or so nearly so as to 

 render the variation not worthy of notice ; hence we will assume them to be of the 

 same thickness. The quantity of matter in them is cooled by the injection from 

 212 to about 150, rarely lower, and in good engines not lower than 170 or 

 180; the mean 160 may be taken for the effect. The specific heat of iron is 

 about 200 times that of steam, and calculating the mass of iron which must have 

 its temperature raised from 160 to between 160 and 212 by each cylinder 

 full of steam, we have the quantity which that of the steam must be lowered. 



The surface of a cylinder is equal to its length, increased by half its diameter, 

 multiplied by its circumference = (/ + | d) d w ; and the thickness, with an allowance 

 equivalent to the escape of heat from the external surface, is one inch and a half 

 = one-eighth of a foot ; and the mass of metal equivalent to the absorption of 

 heat is 



N 



