82 Applications of the Formulas 



serving the previous notation: M=(T x)K' M=Q(x 0) 

 equations which give: , 



K'T+QO .. QK'(T-e) 

 ;r= and M= 



We.find, as in the preceding case, that for heights in feet of: 



3.28 6.56 9.84 13-12 16.40 



the quantities of heat transmitted in B T U per hour per square 

 foot per i Fahrenheit difference, are: 



.338 .816 .305 .301 .297 



For an interior temperature of 59 and an exterior tempera- 

 ture of 42.8, the quantities of heat transmitted are: 



5.48 5.11 4.94 4.87 4.80 



These numbers are smaller than those we found in the preceding 

 case (88 1 ) because the glass is at a lower temperature.* 



883. The two cases which we have just examined, like those 

 relative to the transmission of heat through walls, never actually 

 occur. In the first case the walls opposite the windows are always 

 at a lower temperature than the air, and in the second case there is 

 always a part of the enclosure which is not glazed, and when the 

 heating is effected in part by the radiation from hot surfaces, the 

 rays of heat which fall on the glass increase the quantity of heat 

 which it transmits. But in any case, the quantity of heat actu- 

 ally transmitted is comprised between those which we have com- 

 puted for the two extreme cases. We will return to this question 

 in speaking of the heating of dwellings. 



*The following note from London Engineering, Nov. I, 1902, is of interest here: 

 "In cold countries double glazing is sometimes resorted to, in order to reduce the 

 heat lost from a room to the exterior through the windows. Some experiments made by 

 H. Schoentjes, of Ghent, show that there is a certain distance of separation between the 

 glasses, at which the heat lost is a minimum. The glass used in his experiment was .08" 

 thick and the loss was least when the distance between the opposing sheets was some- 

 where between 2.6" to 4.6". The loss in calm air through one thickness of the glass was 

 at the rate of about .42 B T U per square foot per hour for each degree Fahr. of the dif- 

 ference of the temperature on the opposite side of the sheet. The experiments were made 

 over a range of 12.6 to 40 Fahr. and the rate of loss was somewhat greater as the differ- 

 ence of temperatures increased but the mean was as stated. . With double walls at the 

 best distance apart the rate of loss was about halved. Wetting the outer surface of the 

 glass increased the loss about 39 per cent; whilst if at the same time, a current of air was 

 directed over the outer surface the rate of loss was still further increased up to about .93 

 B T C/per square foot of glass per hour. The utility of the second layer of glass which 

 can be kept dry and in still air would in such a case as this, be very marked.' 



Note that the rate of loss .42 B T U is about a mean between Peclet's calculations 

 for his two extreme cases. The relative value of the double window appears somewhat 

 greater however. 



