CHAPTER III 

 APPLICATIONS OF THE FORMULAS 



861. We have already seen (826) that in designating by M, 

 the quantity of heat that traverses in one hour a plate with paral- 

 lel surfaces, of an area of one square foot, and with its surfaces 

 maintained at the constant temperatures / and /', we have 



M= C -S<> ....... (a) 



In this expression e represents the thickness of the plate in 

 inches and C the conductivity, that is to say the value of Mfor 

 t-t'=i F and e=i inch. 



862. If the body were formed of two superimposed plates, 

 of thicknesses e and e' and conductivities C and C', designating 

 by the common temperature of the surfaces in contact, we have 

 evidently when the regime is established: 



n . C(t-e) n , C'(e-t') 

 M=- - -and M= -L, - 

 e d 



Eliminating 6 we have: 



And for any number of plates: 



863. By means of the tables (859) and the preceding formu- 

 las we may easily compute the quantities of heat which will be 

 transmitted through plates when the temperatures of their sur- 

 faces are known . But these temperatures never are known exactly 

 and can only be measured by very delicate experiments, impos- 

 sible in practical work. Furthermore, in making estimates, it is 

 necessary to have at least an approximate value of the quantity 

 transmitted, in terms of the temperatures of the air inside and out- 

 side of the surfaces. 



864. Consider first a room enclosed by walls of which one 

 only is exposed to the outside air. Let the temperature of the 



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