134 PROCEEDINGS OF THE AMERICAN ACADEMY. 



were used to determiue the change of temperature of water in the heat- 

 conduction apparatus, was 0.0002548 ohm.* With these data we get 

 from the table above : — 



Mean of T 



(1) and (4) 21°. 1 



(2) " (8) S7°.2 

 (6) " (7) 52°. 



(3) " (5) 73°.3 



Here / stands for (L -f- A) X (0.001491 -^ 0.0002548), that is, the 

 length of bridge wire, used in the main experiments, corresponding to 

 a difference of 1° in the temperature of the spirals. Values of / for 

 temperatures intermediate between those here given were found when 

 needed by interpolation between the values of I here given. It will be 

 observed that there is a maximum difference of 11 parts in 1770 between 

 the given values of I. Rigid constancy of I would mean rigid constancy 

 of the mean of the temperature coefficients of the two spirals with ref- 

 erence to the mean scale of the mercury thermometers used. Such con- 

 stancy was hardly to be expected. 



Use op Differential Thermometer in the Main 

 Experiments. 



In the main experiments the plugs bearing the spirals were at first 

 placed in their supporting sockets according to Figure 2, which repre- 

 sents the spiral placed in the incoming stream. The water, therefore, 

 on entering the apparatus, passed the first spiral before reaching its 

 copper connections, but on leaving the apparatus it passed the copper 

 connections of the second spiral before coming to the spiral itself. Ac- 

 cordingly, any net conveyance of heat by the copper wires into or out 

 of the stream was superposed on the action of the conducting disk to 

 produce the difference of temperature noted by the spirals. Any such 

 effect of the copper wires would probably be very small except at high 

 temperatures ; and at such temperatures the effect would be, in large 

 measure, eliminated from the result by the practice, always maintained, 

 of combining one set of observations in which the disk carried heat to 

 the upper stream with another set of observations in which the disk car- 

 ried heat from the upper stream, the mean temperature of the disk re- 



* Each bridge wire was calibrated by finding what length upon it corresponded 

 to the difference between the resistance of a 1 ohm coil and that of a similar coil 

 shunted by a known, niuch larger, resistance. 



