802 PROCEEDINGS OF THE AMERICAN ACADEMY. 



to have been 1° C, and assuming, for the moment, the same rate of sur- 

 face transmission which we have used above, we get, as the amount of 

 heat passing per second between the parts considered and the jacket, 

 300 X 0.000252 = 0.0756 units. This is, jjerhaps, rather more than 

 1% of the heat carried from face to face through the disk, and if it were 

 a fair estimate of the actual transmission between the jacket and the 

 surfaces considered, the neglect of this transmission would make K, as 

 calculated, about 1 % too large. This error would not be eliminated by 

 combining sets of observations, some with the warmer stream above, and 

 some with the colder stream above. But in this estimate, as in that 

 relating to the action at the curved surface of the disk, the rate of 

 transmission assumed is no doubt much too large, the surfaces enclosed 

 by the jacket being, for the most part, well wrapped with cotton. 



It seems, therefore, unlikely that any considerable error was made 

 by neglecting the intercliange of heat between the water jacket and the 

 apparatus within it. 



There is little doubt that much more concordant values of K than 

 those given in this pa])er can be obtained by a somewhat more careful 

 control of the temperature of the water, and by making each set of 

 observations longer than the sets, often very brief, which were made in 

 the investigation which has here been described. 



Summary. 



The thermal conductivity of the cast iron used is about 0.1490 at 

 30° C. The temperature coefficient of thermal conductivity, if Winkel- 

 mann's rule for the change of specific heat of water with temperature is 

 correct, appears to be about —0.00075 between 20° and 75°, so that 

 a rise of about 13° C. corresponds to a fall of \% in conductivity. 



If the change of specific heat of water between 30° and the higher 

 temjjeratures up to 75° were neglected, the value found for the temper- 

 ature coefficient would be about —0.0010. 



The electric conductivity of this cast iron is about 112,200 in c. g. s. 

 units. (See Appendix II.) 



The temperature coefficient of its electric conductivity between 17° and 

 67° is about —0.00118. 



The method used appears to be capable of giving better results than 

 have yet been obtained by it. 



