3"0 EXTRACT ON WARMING BUILDINGS EY HOT WATER. 



surrounding medium. The subject of the radiation of heat, and the 

 rate at which a heated body cools, under various circumstances, will 

 be fully considered in another chapter. But for temperatures below 

 the boiling point of water, and under such circumstances as we are 

 now considering with regard to hot-water pipes, the velocity of 

 cooling may be estimated simply in the ratio of the 'excess of heat 

 which the heated body possesses above the temperature of the sur- 

 rounding air. The variation in the rate of cooling, arising from a 

 difference of the superfices to the mass, is, for bodies of all shapes, 

 inversely, as the mass divided by the superfices. Therefore the 

 relative ratios of cooling, for any two bodies of different shapes and 

 temperatures, is the inverse numbers obtained by dividing the mass 

 by the superficies, multiplied by the direct excess of heat above the 

 surrounding air ; provided the temperature of the heated bodies be 

 below 212°. Thus, suppose the relative ratio of cooling be required 

 for two cisterns filled with hot water, one a cube of 18 inches, at the 

 temperature of 200°; the other a parallelopiped, 24 inches long, 15 

 inches wide, and 3 inches deep, at the temperature of 170°; the sur- 

 rounding air in both cases being 60°. Then, as 



Inches. Inches. 



The cube contains . . . 5832, divided by 1944, the superficies = 3*0 

 The parallelopiped contains. 1080 ditto 954 ditto =1*13 



The inverse of these numbers is, to call the cube 1*13, and the 

 parallelopiped 3*0. Then multiply 1 • 13 by 140 (the direct excess 

 of temperature of the cube), and the answer is 158*2 ; and multiply 

 3*0 by 110 (the direct excess of temperature of the parallelopiped), 

 and the answer is 330 '0. Therefore the parallelopiped will cool, in 

 comparison with the cube, in the proportion of 330 to 158, or as 

 2*08 to 1 ; so that, if it requires two hours to cool the cube, a half, 

 or a quarter, or any other proportional part of its excess of heat, the 

 other vessel will lose the same proportional part of its excess of heat 

 in one hour. 



" It is evident that these different velocities of cooling are quite in- 

 dependent of the effect that the respective bodies will produce in 

 warming a given space ; for as the cube contains upwards of six 

 times as much water as the other vessel, so it would warm six times 

 as much air, if both vessels were of the same temperature. " But if 

 six ]of the \>blong vessels were used, they would heat just the same 



