10 Br. W. S. Day on a Comparison of 



In order to fulfil the second requirement the tank was 

 placed inside a larger wooden box whose walls were every- 

 where about 5 centhn. from the sides of the tank, and the 



Fig. 2. — Comparison-Tank (cross-section). 



intervening space was filled with cotton-wool. This protected 

 the tank very well, except at the top, from too rapid a gain or 

 loss of heat from the air when this was at a different tempera- 

 ture from that of the tank. The layer of cotton did not 

 extend quite up to the top of the tank. The non-conducting 

 layer above this point had to be easily removed to allow 

 the removal of the lid. A covering of felt was therefore con- 

 structed about 2 centim. thick, which extended over all the 

 parts of the tank not protected by the cotton, except the glass 

 window. This had to be exposed, at least in part, to allow 

 of reading the thermometers. In order still further to pro- 

 tect the thermometers from any change of temperature, the 

 bulbs were placed inside a brass box, as shown in the sections. 

 This box was 10*5 centim. long, 8 centim. wide, and 5 centim. 

 deep. It was so constructed that two lids on the top and two 

 on the bottom could be opened or closed from the outside of 

 the tank. While the water was being stirred, in the manner 



