202 Prof. L. Hill, Messrs. H. M. Vernon, and D. Hargood-Ash. 



where H' was the heat lost per second per square centimetre from the wet 

 "kata," v the wind velocity, F the maximum vapour pressure at 36 - 5° C, 

 / the actual vapour pressure of the air at the time of the observation, equal 

 to the difference between 36-5° C. and the air temperature, and a, I, c, and d 

 numerical constants. This was afterwards modified to the form 



H' = (a' + V y/v) 6 + (c' + d' v ' 3 ) (F -/)*/ 3 , 



which was given in the later paper. 



Our new investigations give an equation of the form 



H' = (a" + b" x / v )$ + {c" + d"yv)(F-f)W 



but, just as for the dry "kata," two equations are found to be necessary, one 

 for velocities greater than 1 metre per second and one for velocities less than 

 this. These equations are 



H' = (013 + 047 y/v)6 + (0-035 + 0-098 j/v){Y-f)W (iii) 



for velocities greater than one metre per second, and 



H' = (0-20 + 0-40 ^^ + (0-060 + 0-073 ^)(F-/) 4 / 3 (iv) 



for velocities less than 1 metre per second. 



In each case the first term on the right-hand side of the equation repre- 

 sents the dry " kata " heat loss. 



These expressions are cumbersome for practical use, so that when the 

 question was re-investigated we proposed to try if a formula in which the 

 humidity was represented by the wet bulb temperature would give satis- 

 factory results; that is to say, a formula of the form 



H' = (a + bv x )6', 



where 6' is the difference between 36-5° C. (97'7° F.) and the wet bulb tem- 

 perature, t' , a and b being numerical constants, and x some power to which 

 the velocity, v, had to be raised. 



When our experimental values of H'/0' were plotted against the wind 

 velocity values, as shown in the upper curves of figs. 1 and 2, it appeared that 

 such a relation existed between the variables, and that, considering the whole 

 range of velocities from zero up to 17 metres per second, the value x — 1/3 

 gave satisfactory results. 



We may, therefore, write an empirical wet " kata " formula in the form 



H' = (a+bj/v)?, 



where H' is the heat loss per second per square centimetre, v the wind 

 velocity, and 6' the difference between 36-5° C. (97'7° F.) and the wet bulb 

 temperature, t'. As before, two equations are necessary to cover the whole 



