446 THE REGULATION OF TEMPERATURE 



In the assessment of the conditions of the atmosphere making for ideal 

 heat-loss, two instruments are in common use, viz., the wet and dry bulb 

 thermometer and Hill's " Kata " thermometer (or its popular modification — 

 the Comfimeter). The first of these instruments consists of two similar 

 mercury thermometers fastened side by side on a stand. The reservoir 

 of one is covered by a close fitting muslin bag which is kept moist by connec- 

 tion to a wick dipping in water. The water evaporates from the bulb at a 

 rate depending principally on the degree of saturation of the atmosphere with 

 aqueous vapour. It is obvious that, if the air is already saturated with 

 moisture and no evaporation is taking place from the wet bulb, both thermo- 

 meters will register the same temperature. On the other hand, if the difference 

 in the temperatures recorded is great, one may consider that the air is dry. 



Example. The degree of humidity of the air may be calculated from the 

 formulae, 



/ 

 degree of humidity — — and/ = F — AH{t — 9), 



where A = the constant of the instrument, 



F = the tension corresponding to 6°, the reading on the wet 



bulb thermometer, 

 H = atmospheric pressure, 

 t = reading on dry bulb thermometer, 

 and »i = maximal vapour tension at t° C. 



If ^ = 0-00082, t = 20° C, 9 = 16° C, H = 758 mm. Hg, 



F (from a table of vapour tensions; = 13-63 mm. Hg. 

 Then / = 13-63 - 0-00082 x 758 x 4 = 11-16 mm. Hg. 



The maximal vapour tension at 20° C. (from a table) = 17-52 mm. Hg. 



Therefore, the degree of humidity = 7" = T^Tm ~ 0"637. 



Valuable as this information is, it does not give us a true idea of the cooling 

 power of any atmosphere. It fails to register the cooling due to air move- 

 ment, for instance ; so L. Hill devised the kata-thermometer, whereby the 

 rate of cooling is measured directly. 



The "Kata" thermometer is a large bulbed spirit thermometer with 

 only two marks on the scale indicating a difference of 5° F. in their difference 

 of level. That is, the top mark = 100° F. and the lower mark = 95° F., 

 with a mean value about the mean value of body temperature = 97-5 F. 

 or 36-5° C. The instrument has inscribed on its stem a factor F, relating 

 principally to the cooling area of the bulb. The kata-thermometer is placed 

 in warm water (about 105° F.) till the spirit rises and enters the top bulb. 

 The bulb is now wiped dry and suspended well away from the body or any 

 other source of heat. A stop-watch is started when the meniscus passes the 

 top mark and stopped when it reaches the lower mark. Three to five readings 

 are taken. The average time in seconds occupied by the meniscus in falling 

 from 100° to 95° F. divided into F gives the cooling power of the atmosphere 

 in that place due to conduction and radiation in millicalories per cm.^ per sec. 



To determine the cooling power due to the evaporation of moisture a little 

 silk-net finger-stall is fitted on the bulb and the experiment carried out as 

 above. Excess water, of course, is removed from the finger-stall. Suppose 

 F = 470, the dry reading = 78 seconds, and the wet reading = 26 seconds, 

 then the loss of heat due to radiation, conduction and convection is just 

 over 6 millicalories per square centimetre per second. The humidity of the 



