OF ARTS AND SCIENCES. 51 



The units in which the preceding conductivities are expressed are 

 the centimeter, the minute, a centimeter deflection on the galvano- 

 meter scale, and the unit of heat is the amount of heat required to 

 raise the temperature of 1 cm.^ of the glass one unit of deflection. 

 To compare these conductivities with those determined hy Forbes for 

 iron, we must reduce them to functions of the units : the foot, the cen- 

 tigrade degree, and the minute. 



/iS and D^t are ratios, and are independent of units. A contains 

 cm.'^ and " area " contains cm.* 



1 cm.2 = .155 sq. in. =: .00107 sq. ft. 

 1 cm.3 = .061 cu. in. = .000035 cu. ft. 



In determining the relation between one degree centigrade and one 

 unit deflection, I made four measurements, using heated sand, the 

 thermometer and thermopile being buried side by side. 



5.5 cm. corresponds to a rise in temp, of 12.0 

 8.5 « " " 16.3 



5.0 « « « 10.8 



6.7 « « « 14.2 



From these results, evidently, 



o 



1 cm. corresponds to a rise in temp, of 2.18 

 1 « « « 1.91 



1 "^ « « 2.16 



1 «' « " 2.11 



4)8.36 

 Average 2.09 



Hence, to reduce K to the required system of units, we divide by 

 2.09 and .00107 and multiply by .000035, or we multiply by .015. 



The conductivities determined before become in the new system of 

 units, 



K= .00270. 

 K= .00315. 

 K= .00420. 



These results can be summed up as follows. The numbers in the 

 first row give, in centigrade degrees, the excess of the temperature of 

 the bar above that of the air, and the second row the corresponding 

 conductivities. The air was about 21° C. 



