8o Applications of the Formulas 



Direct experiments have given, 



i .90 .75 .67 .60 



If we create a vacuum between the envelopes, the transmis- 

 sion of heat will only take place by radiation, and the quantity 

 transmitted may evidently be deduced from the preceding for- 

 mula, by making C therein equal to zero. It then becomes, 



Assuming the envelopes to be of zinc, we would have K= 

 ^=.492; supposing their number to be ten, and taking A"=.82, 

 /=2i2, /"=59 we find M=.jS. To compare this transmission 

 with that that would take place if the interval between the vessel 

 and the outermost envelope were filled with eiderdown, one of 

 the poorest of conductors, assume the interval to be .4". The 

 formula for the transmission of heat through a plate is 

 C Q ( t t" }* y in which C is the conductivity of the material, 



C-\-Qe which for eiderdown is .29, Q the loss of heat 

 from the outer surface which is here .87, and e the thickness 

 which we have taken as .4". For these numerical values we find 

 the quantity of heat transmitted to be equal to 68.2 nearly ninety 

 times greater than with the envelopes and vacuum. 



The means that I have just indicated for diminishing the 

 transmission of heat is the most efficacious that I know. To 

 apply it to two concentric metal cylinders it is necessary to close 

 both ends of the interval separating them with poorly conducting 

 material, then to solder near one end a very small lead pipe, 

 which serves to form the vacuum and is then closed by squeezing 

 it together and melting off the end in the flame of a blow pipe. 



This arrangement would be especially advantageous in appa- 

 ratus destined to make ice. 



TRANSMISSION OF HEAT THROUGH WINDOWS 



880. We will examine two cases of the transmission of heat 

 through windows ; the first will be the same as the case first con- 

 sidered of transmission through a wall, and, the second the case of 

 an enclosure formed entirely of glass. 



*Note that in this case we know the temperature of the inner surface of the wall 

 and of the outer air, and thus M= (tf) and M=Q (t't"). Combining these we get 

 the equation given above. 



