Applications of the Formulas 75 



i '^ M=Q(t'0) M=K' (Tt) 



From these equations; 



Q(eICT+Ce)+CK' T 



Q(eK' Q+CQ)+CK' T 



K'CQ(T-e) 



871. If the wall was made up of two walls, in immediate 

 contact, of thicknesses e and e' and conductivities C and C' we 

 would have: 



e e' 



M= Q ( t' 0) M=K[ (T t) 



and consequently: 



K'Q(T-V) 



If there were any number of walls the general formula 

 would be: 



M= ^Q(T-o) 



Q + K'+K' i 



Applying this case to a wall as in 868 with C= 13.71, 

 ^=.737 and A"=.4o, Q=\.\y] 7^=59 and 0=42. 8 we get 

 the results shown by the dotted lines of figure 1 2 . 



It is useful to notice that if the walls had a height of 66 feet 

 the value of A"' would be .39 instead of .40 and we would obtain 

 practically the same numerical results. 



872. The values of ^/obtained in this last case are much 

 smaller than in the first; this is due, as already explained, to the 

 much lower temperature of the interior surface of the walls. 



873. I would remark that, in the two cases just examined, 

 it is necessary to take some precautions in measuring the temper- 

 ature ; if the thermometer were exposed freely to the air, the tern- 



