The Cooling and Evaporative Powers of the Atmosphere. 443 



through the tube at velocities which could be varied at will. Two gauze 

 screens were fixed in the tube to ensure an equable flow of air, and suitable 

 side openings provided through which (1) the kata, (2) a dry bulb, (3) a wet 

 bulb thermometer, were inserted in order, so that the kata came first and the 

 wet bulb nearest the fan. The thermometers were read immediately before 

 the kata was introduced in each observation of cooling rate. 



The whole apparatus was placed in a small chamber, kindly placed at our 

 use by Mr. H. K. Davis, at the works of Siebe, Gorman, Ltd., a chamber in 

 which we could at will vary the temperature and humidity through fairly 

 wide ranges. To determine the velocity of the wind we used the reading of 

 the dry kata and our new formula H/d — 0'27 + 0*49 ^/V, taking dry kata 

 readings alternately with wet kata readings in each series of determinations. 



To determine humidity we used the readings of the wet and dry bulb 

 thermometers and the tables given by the Eoyal Meteorological Society. 

 Our results are given in Table 3, and are plotted out in G-raph II, and the 

 formula which best fits these appears to be 



E/(F-/)4/3 = 0-85 + 0-102 V»-3 



where E = the cooling power due to evaporation, F = the vapour tension 

 in mm. Hg. at 36'5° C. (the mean temperatm-e of the kata surface), /= the 

 vapour tension of the air, and V = the velocity of movement of the air in 

 metres per sec. The full formula for the wet kata is then 



H = (0-27 + 0-4VV)^ + (0-85 + 0-102 V»-3)(F-/)V3 



The method available to us only allows an approximate fit to be obtained, 

 firstly, because the velocity is not measured at the same time as the cooling 

 of the wet kata, but just before and after, and the electric power driving the 

 fan may vary slightly ; secondly, because the relatively sluggish thermo- 

 meters may not indicate slight changes in air temperature and humidity, 

 which the excessively sensitive wet kata responds to ; thirdly, because the 

 tables, from which the vapour tension is deduced from wet and dry bulb 

 temperatures, are admittedly only approximately correct ; fourthly, because 

 of errors in time, the wet kata cooling very rapidly. Error might also arise 

 if too thick a cover was used for the wet kata, and an excess of hot watei 

 retained around it. We avoided any such error by using a muslin finger 

 stall. After warming in the Thermos flask, excess of water was removed by 

 gripping the glove above the bulb and drawing it up tightly, and giving the 

 kata one or two downward shakes. Using these means, successive readings 

 of the wet kata gave closely concordant results. 



We cannot claim that the formula allows us to determine correctly either 



