94 Applications of the Formulas 



sudden oscillations, that whatever the law according to which the 

 wall cooled during the first part of the winter, and heated during 

 the second part , the quantities of heat emitted and absorbed by 

 the wall, could have but a slight influence on the transmission, 

 according to the hypothesis of a constantly maintained transmis- 

 sion . We see furthermore that during the decrease of the exterior 

 temperature the cooling of the walls diminishes by a small amount 

 the quantity of heat which must be furnished in order to main- 

 tain the interior temperature, and that during the increase of the 

 exterior temperature, there will be more heat to be furnished to. 

 reestablish the original regime throughout the wall. 



903. According to what we have just said, the curve of 

 mean monthly temperatures of the heating months presents but 

 one minimum; but each day there are several variations in oppo- 

 site directions so that the actual curve of temperatures shows a 

 great number of sinuosities around the curve of mean temperature. 

 These variations act directly on a heated room through the win- 

 dows, because the windows take almost instantaneously a tem- 

 perature which is a mean between the interior and exterior tem- 

 peratures. It is otherwise with the walls; they furnish, when 

 the exterior temperature falls, a certain quantity of heat, and 

 when the exterior temperature rises again to the original point, 

 they absorb the same quantity of heat, so that the quantity of 

 heat to be furnished to maintain a constant interior temperature, 

 varies much less rapidly than the exterior temperature. Since 

 these variations are equal and of opposite signs in regard to the 

 curve of mean temperature, in whatever way the partial cooling 

 and heating of the wall takes place, the losses and gains end by 

 compensating one another, and the total expenditure of heat dur- 

 ing the heating season remains the same as if the exterior tem- 

 perature had followed the curve of mean monthly temperatures, 

 or as if the exterior temperature had remained constant at the 

 mean temperature for the whole season. This is confirmed by 

 experience. 



904. The phenomena produced in walls by sudden varia- 

 tions of the exterior temperature are very complicated. If a fall 

 of temperature takes place there is an increase of loss from the 

 exterior surface, and a decrease in its temperature which spreads 



