404 ILLUSTRATIONS OF THE DYNAMICAL THEORY OF OASES. 



where J is the mechanical equivalent of heat in foot-pounds, and q is the 

 transfer of heat in thermal units. 



Now MN=p, and l ~r- . so that 



Also, if T is the absolute temperature, 



1 dT2 



(60), 

 where jp must be measured in dynamical units of force. 



Let /=772 foot-pounds, j> = 2116 pounds to square foot, ^ 

 v=1505 feet per second, 7*= 522 or 62 Fahrenheit; then 



where 5 is the flow of heat in thermal units per square foot of area ; and T' 

 and T are the temperatures at the two sides of a stratum of air x inches thick. 



In Prof. Eankine's work on the Steam-engine, p. 259, values of the therum/ 

 resistance, or the reciprocal of the conductivity, are given for various substances 

 as computed from a Table of conductivities deduced by M. Peclet from experi- 

 ments by M. Despretz : 



Resistance. 



Gold, Platinum, Silver .......... ,.0'0036 



Copper ................................ Q'0040 



Iron .................................... O'OOOG 



Lead .................................. 0*0198 



Brick .................................. 0-3306 



Air by our calculation ............ 40000 



It appears, therefore, that the resistance of a stratum of air to the con- 

 duction of heat is about 10,000,000 times greater than that of a stratum of 



