Emission and Transmission of Heat 



35 



of iron filled with mercury are employed, an agitator is no longei 

 necessary, and a thermometer with a long reservoir inserted in the 

 vessel, gives its temperature accurately. 



785. Figure 163 shows the little apparatus 

 used in reading the thermometers, ab is a small 

 plate covered with white paper, and bearing at 

 the middle of its height two projecting rods, one 

 on each side of the 

 stem of the ther- 

 mometer. These 

 rods are parallel to 

 one another and 

 perpendicular to 

 the plate ab. They 

 have fastened upon 

 them two hairs 

 which determine a 

 plane perpendicu- 

 lar to the stem of 

 the thermometer 

 and in which plane 

 the eye of the ob- 

 server should be 

 placed. 



c and d are two rings through which the 

 stem of the thermometer passes. They are lined 

 with cork which can be more or less compressed 

 by thumb screws. 

 This apparatus is necessary on account of the motion caused 

 by the agitator which renders the use of a cathetometer impos- 

 sible. 



786. Figure 164 shows the apparatus used to refill the ves- 

 sels at certain stages of their cooling, which is necessary in order 

 that the cooling surface may remain constant. 



An opaque vessel, in which the level of the water has de- 

 scended, has to be refilled without taking it out of the constant 

 temperature chamber and without spilling any water. 



The apparatus consists of a glass tube AB open at both ends 



Fig. '59- 



Figs. 157 and 158. 



