determining the Thermal Conductibility of Bodies. 135 



turning the cock b, which in the position b 1 allowed steam, and 

 in the position b q cold water, to reach the bar, the section of which 

 is indicated in the figure by a. 



<M4 



By a special investigation the following relations were obtained 

 between the divisions of the thermometers used : — 



logw 1 ° = log77 4 °-0-18253- 

 log n 1 ° = log7i 2 ° + 0-02120 



logn 8 °=log7i 4 °-0-20373 J> (10) 



log n 3 ° = log n 2 °- 0-03685 

 logrc 3 ° = log V-0-01565. 



in which n { ° denotes the n° of the thermometer No. 1, n 4 ° the 

 n° of the thermometer No. 4, and so on. 



The absolute temperature of the bar needs only be known in 

 so far as it is necessary to calculate the mean temperature, that 

 is, the temperature to which the final value of k refers ; other- 

 wise it is enough to know the relative values of the degrees of 

 the thermometer, and even this is not necessary if in the obser- 

 vation the places of the thermometers are changed. 



By a similar change another source of error is avoided. For 

 as glass is a bad conductor, it may be foreseen that the ther- 

 mometers do not instantaneously and completely indicate the 

 temperature of the bar at a given moment ; it may, however, be 

 assumed that these small deviations will occur in an entirely 

 analogous manner when both thermometers are simultaneously 

 observed, and that if, in consequence of the unequal form, mass, 

 &c. of the thermometer-bulbs, a difference occurs, this can be 

 eliminated by the exchange just mentioned. A third source of 



