188 ROYAL SOCIETY OF CANADA 



We have endeavoured by a process of experimentation, e.g., by alter- 

 ing the size of the thermometer bulbs, using different kinds of wick, 

 changing the position of the reservoir; varying the temperature of the 

 feed water, and using currents of air at different speeds, to find out 

 under what conditions the readings of the wet and dry bulb instrument 

 are approximately true. 



As will be seen, we have obtained some results which promise to 

 reward further investigation ; but as will be readily recognized, this must 

 be a question of time, in order to collect sufficient data on which to form 

 trustworthy conclusions. 



Comparisons were made, in the first instance, with three wet and dry 

 bulb insti-uments, set up in such a way that a draught of air could be 

 drawn across them by means of a motor fan. 



Our original instrument had a rather small spherical bulb, and a 

 comparatively large wick. Our second instrument had a much larger 

 spherical bulb, and a rather smaller wick; while our third instrument 

 had a cylindrical bulb, about 2 cms. long, and a wick attached to the 

 lower end. 



The bulbs, in all cases, were covered with cheese-cloth, which had 

 previously been chemically treated to remove fats, etc. Chemically 

 treated silk was used at first in place of the cheese-cloth, but the results 

 were not so satisfactory. We believe the coarse structure of the latter, 

 which exposes the bulb to view in the meshes, is more advantageous than 

 the closely woven silk, in that it gives greater sensitiveness. 



Ample reservoirs of distilled water were provided, and the length of 

 wick above the water varied. This did not appear to make much differ- 

 ence provided there was always a sufficient supply of water to keep the 

 entire bulb wet. 



The need of having a continuous current of air about the bulb was 

 shown at the outset, and in this respect we have established otie 

 essential condition, viz., the rate, provided there is a perceptible current 

 does not matter whether it be fast or moderately slow as long as the 

 bulb be kept moist. With the higher rates of speed this is rather diffi- 

 cult, the upper part of the wick tends to dry quickly. On the other 

 hand, there is obviously a minimum limit, which we have not been able 

 to determine as yet; but we have shown that a stationary atmosphere 

 does not give correct readings owing to diminished evaporation. 



The following table brings out this point very well : — 



TABLE I. 



