26 



Mr. C. H. Lees on 



From the approximate value of k^ which can be 

 obtained from the 5th and 6th column of this table, 



the approximate value of ^/^^-j- when x=o is determined 



to be 41. Hence column 7 is calculated ; and the numbers 

 in it are 



X 



a:=0 



The numbers in the column headed k^ are calculated 

 from the equation 



ph{f{v - V)"W^ + 41 } f(y _ y)i 17^0; + 41 



K = 2 , =-000,265^ 



dv 



dx 



dv 

 dx 



Conductivities of Crown and Flint Glass. 



Taking -250 as the conductivity of the brass bars the 

 following results are deduced from several experiments on 

 crown and flint glass discs, the temperatures in each disc 

 being supposed represented by the equation v = ax-\-bx^^ the 

 origin of x being taken at the cooler surface of the disc and 

 the temperature of that surface being taken as zero. 



Crown glass '16 cm. thick. 



X 



V 



1c ^^ 



dx 







•16 







t6*02 



•222 



•250 



dv 

 k^-j- being the amount of heat which crosses a surface 



of contact of disc and bars, as calculated from the observa- 

 tions of temperatures in the bars 



From this I find k for crown glass = •00235. 



