The Wet-Bulb and Kata Thermometers. 121 



In the course of observations, I began to detect some discrepancies 

 between the wet-bulb reading and the state of discomfort of the 

 body. The most striking of these occurs when a hot day with 

 strong north wind and clear sky undergoes a "change." The sky 

 becomes overcast and the wind drops. There may or may not be 

 electrical disturbance. At this time the air conditions may become 

 most oppressive, and sweating may be very copious on exertion. 

 Now, in all the observations I have made in five summers, I have 

 only noted two occasions in which the wet-bulb rose with the over- 

 casting of the sky and the droj) in the wind. In all other cases 

 the wet-bulb, like the dry-bulb, fell. I, therefore, came to the con- 

 clusion that the Avet-bulb is not sufiiciently sensitive to air currents. 

 Experiments on the evaporation from the skin and from an eva- 

 porimeter confirmed me in this view.^ I, therefore, devised a wet- 

 bulb thermometer, having its bulb within a cylindrical gauze cage 

 open at the bottom but otherwise covered with cloth. At the sug- 

 gestion of Professor Roaf bolting cloth of standard mesh was used 

 for this latter purpose. These "jacketed wet-bulb thermometers" 

 manifested a much better response to air movement, and when 

 tested in a " change," usually gave a rise in temperature. Further 

 experience, however, brought to the light new difficulties. The cloth 

 was apt to get greasy, and to shrink, so I fell back on various porous 

 substances. The same trouble with grease still pursued me, and, 

 most annoying of all, I found that the slightest shift in the position 

 of the bulb made a change in the height of the column, and that it 

 was practically impossible to obtain two jacketed thermometers that 

 gave the same reading. This particular effort, then, which has cost 

 me some considerable time I have been compelled to give up, as I 

 do not see how standardisation can be effected. 



The Kata-thermometer of Leonard Hill works on a different prin- 

 ciple. Here the time of cooling of a large wet-bulb through a range 

 of lOo F. (from lOQo F. to 90^ F.) is taken, and the cooling power 

 of the air calculated in terms of millicalories per cm^ per second. 

 I have had just one summer's experience with three of these instru- 

 ments, one of them having been kindly sent to me by Professor 

 Leonard Hill who had worked out its special factor.- The Kata- 

 thermometer certainly indicates well the onset of oppressive thunder 

 weather and therein manifests its superiority to the wet-bulb. But 

 I soon discovered that in its present form it is somewhat too sensi- 

 tive to air currents, at any rate in hot, dry weather. The following 



1 W. A. Osborne. Contributions to Physiol ojrical Climatology, loc. cit. 



2 A kerosene tin can be employed with very fair results for obtaining the readin;,' in still air. 



