The Kata-ihermometer as a Measure of Ventilation. 203 



range. The constants a and b, when v is in metres per second, H' in milli- 

 calories per second, and 6' in degrees Centigrade, are given hy the following 

 two equations : — 



H' = (010 + 1*10 v) 6' for velocities greater than 1 metre per second, (mo.) 



H' = (0"35 + 085 ^/ v) 6' for velocities less than 1 metre per second. (iva) 



Our results with the wet "kata " in winds are only over a limited range of 

 humidities (the wet bulb temperature varying from 9 o° to 19 - 8° C), owing to 

 the difficulty of controlling the humidity in the large rooms in which the 

 wind tunnels are situated. In the case of still air, however, it is possible to 

 obtain more varied conditions, and considering the still air wet " kata " 

 formula 



H' = O-270 + O-O85 (F-/) 4 / 3 , (v) 



given in the original paper, it seems probable that for very dry or very moist 

 air, equations (iii) and (iv) are more correct than (iiia) and (iva). It is not 

 possible to find an equation for still air of the form 



H' = a&. 



This can be seen at once by putting in fixed values of the wet bulb tempera- 

 ture in (v) and varying the humidity. In this way it is found that a varies 

 from - 5 to 0*7 as the temperature and humidity vary from 10° C. and 

 10 per cent, relative humidity to 20° C. and 100 per cent, relative humidity. 

 For any given wet bulb temperature the value of a increases with the 

 humidity, the increase being much more rapid at' low relative humidities. 



With respect to the wind velocity index, when considering evaporation, we 

 find that observers have come to many different conclusions on the matter. 

 John Dalton gives the value as unity. A number of late investigators found 

 it to be - 5.* Eecent observations made by Bigelow,f in which he deter- 

 mined the rate of evaporation of water from pans 2 to 6 ft. in diameter in 

 wind of various known velocities, gave results agreeing with Dalton's. It 

 may be pointed out that for certain ranges of wind velocity, e.g., between the 

 velocities of 0'3 and 2 - 5 metres per second, our results hold for a value of 1 

 for this index, but many more than two equations would be necessary to 

 cover the whole range of our experimental work with this value, while for the 

 index of 1/3 very close agreement is obtained by the use of two equations 

 of the same form. The velocities recorded by Bigelow were mainly low. It 

 can hardly be expected that the conditions of evaporation from large vessels 

 and from such instruments as the " kata," where the evaporation surface is 

 vertical and the water only a thin film, should be identical. 



* Cf. Harm, ' Lehrbucli der Meteorologie,' Ed. 3, 1915. 

 + Bigelow, ' Monthly Weather Eeview,' 1910, p. 307. 



Q 2 



