32 Prof. Maxwell on the Process of Diffusion of t too or 

 due to the motion of the centre of gravity, or 



where ft is a coefficient, the experimental value of which is 1*634. 

 Substituting E for M in Prop. XIV., we get for the transference 

 of energy across unit of area in unit of time, 



hi- — ^(j/9M»W), 



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 /= -r-, so that MN/= -r- ; 



T , fiv* dv /K . 



Also, if T is the absolute temperature, 



T dx~v dx' 

 /.%=-f/^lg, (60) 



where p must be measured in dynamical units of force. 



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

 l=^~ inch, v = 1505 feet per second, T=522 or 62° Fahren- 

 heit; then 



T'— T 



9= ioooS (61) 



where g 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. Rankine's work on the Steam-engine, p. 259, values 

 of the thermal resistance, or the reciprocal of the conductivity, 

 are given for various substances as computed from a Table of 

 conductivities deduced by M. Peclet from experiments by M. 

 Despretz: — 



Gold, Platinum, Silver. . 00036' 



Copper 00040 



Iron 00096 



Lead 00198 



Brick 03306 



Air by our calculation . . . 40000 



It appears, therefore, that the resistance of a stratum of air 

 to the conduction of heat is about 10,000,000 times greater than 



