Applications of the Formulas 99 



the wall follow one another according to the regime of cooling ; 

 but, to determine them on the assumption that at the beginning 

 of cooling, the temperatures follow one another according to the 

 regime of transmission, we would be lead to very long calcula- 

 tions, to formulas composed of an infinity of terms and which 

 would not really be of any practical value. However, we may 

 admit the following facts to have been well demonstrated both by 

 experience and calculation. 



i St. When a wall is left to its own cooling, the temperature 

 of its outer surface falls- rapidly, the more so the smaller its con- 

 ductivity and its specific heat. The line of temperatures of the 

 points within the wall changes from the straight line of regular 

 transmission, to a curve intersecting the straight line at distances 

 increasing with the time. The curve approaches more and more, 

 as time goes on, to that curve which calculation would give for 

 the case of regular cooling on the assumption that at the begin- 

 ning of cooling the temperature of the points within the wall 

 follow one another according to the regime of cooling. 



2nd. The quantity of heat transmitted by the wall, result- 

 ing solely from its own cooling, is never more than a very small 

 fraction of that which it would allow to pass in a regular regime 

 of transmission . 



3rd. The quantity of heat lost by a wall in ten hours is gen- 

 erally but a small part of the heat contained in the wall at the 

 beginning of cooling, at least for walls as thick as 19.7 inches; 

 for examplefor T=5g, 0=42.8, ^=50.7, t' =45. 8, J/= 3 . 3 6 

 (871), the quantity of heat above 32 contained in the wall 

 per square foot of surface would be 1.64 X 62.4 X 2.2 X .2 (49 32) 

 =764. Whilst the quantity of heat that would be lost in ten 

 hours by regular transmission would be only 3.36X10=33.6, 

 and the quantity lost by cooling would be much smaller. 



4th. Cooling, during suspension of heating, results princi- 

 pally from the transmission through the glass of the windows 

 and the infiltration of air through the cracks of doors and win- 

 dows when the room is provided with a chimney. 



TEMPORARY HEATING OF A ROOM 



910. When a room is only to be occupied for a very short 



