EXTRACT ON WARMING BUILDINGS BY HOT WATER. 



301 



quantity of air as the cube ; but the latter would require rather more 

 than 2i hours to do what the oblong vessels would accomplish in 

 one hour, supposing the temperature to be the same in both cases. 

 In the previous example the temperatures are supposed to be dif- 

 ferent ; otherwise the relative ratio of cooling of the two vessels 

 would have been as 2£ to 1, instead of 2 to 1, as stated. 



" In estimating the cooling of round pipes the relative ratio is very 

 easily found ; because the inverse number of the mass divided by the 

 superficies is exactly equal to the inverse of the diameters. There- 

 fore, supposing the temperature to be alike in all, 



If the diameter of the pipes be . . 1. 2. 3. 4 inches, 

 The ratio of cooling will be . . .4. 2. 1'3 1 



That is, a pipe of 1 inch diameter will cool four times as fast as a 

 pipe of 4 inches diameter ; and so on with the other sizes. These 

 ratios, multiplied by the excess of heat which the pipes possess above 

 that of the air, will give the relative rate of cooling when their tem- 

 peratures are different, supposing they are under 212° of Fahren- 

 heit ; but if the temperatures are alike in all, the simple ratios given 

 above will show their relative rate of cooling, without multiplying by 

 the temperatures. When the pipes are much above 212°, as, for 

 instance, with the high pressure system of heating, the ratio of 

 cooling must be calculated by the rules given in the IXth Chapter. 



" The unequal rate of cooling of the various sizes of pipes renders 

 it necessary to consider the purpose to which any building is to be 

 applied that is required to be heated on this plan. If it be desired 

 that the heat shall be retained for a great many hours after the fire 

 is extinguished, then large pipes will be indispensable ; but if the 

 retention of heat be unimportant, then small pipes may be advan- 

 tageously used. It may be taken as an invariable rule, that in no 

 case should pipes of greater diameter than 4 inches be used, because, 

 when they are of a larger size than this, the quantity of water they 

 contain is so considerable, that it makes a great difference in the 

 cost of fuel, in consequence of the increased length of time re- 

 quired to heat them. For hothouses, greenhouses, conservatories, 

 and such like buildings, pipes of 4 inches diameter will generally be 

 found the best; though, occasionally, pipes of 3 inches diameter may 

 be used for such purposes, but never any of a smaller size. In 

 churches, dwelling-houses, manufactories, &c, pipes of either 2 or 



