of some Rocks at Higli Temperatures. 53 



couple, it is always found that when an approximately 



steady state is reached -jr is very small compared with -_; 



so we may assume that the rate o£ change of temperature of 



every part of the rock is approximately -jr. So if Q calories 



be generated per second per centimetre along the axis and C 

 be the thermal capacity of the rock per unit volume, we have 

 for a distance p from the axis 



_ ^dd n 2 dO , 



_d0 = Q _ cp dO 

 dp 2ttK p 2K ' dt' 



So a__Q_w **- <***-*!*) dO 



2TrK° 8e r 1 4K 'dt' 



Now for the limestone cylinder, 



^ = 0*97 cm. 

 r 2 = 3*18 cm. 

 c =0*53 about. 



If A be the current in amperes flowing through the 

 central wire and V be the P.D. in volts across the ends of 

 the cylinder, W = AV is the energy liberated in watts inside 



W 



the cylinder, and Q= - __ — — rr, since the cylinder is 

 J ^ 13'67 x 4*18' J 



13*67 cm. long. Putting in these figures we obtain 



K= x 1.0 3 , 



where a = the rate of rise of average temperature in degrees 

 per minute. We may write this equation 



K=3-304x-^-xl0- 3 , 



where W X = W — 6' 15a, and maybe regarded as the effective 

 watts. In applying this correction two sets of readings of 

 W, 0, and A are taken, and the mean values of W and A 

 taken and used in the above equation. The value found in 



