6< q Mr. A. E. McLeod ov the 



alone suffices we have approximately (see discussion of 

 equation (50)) 



u-:u e- a ^ f >' cK —u e- 2ht l c P< r , .... (55) 



since &== \'2R when H is small. 



For a spherical bulb, when the first term of the series- 

 suffices, the cooling equation, which is 



^^S v{<-fH(H-i)r • • • (56> 



becomes 



ur=ni e- a2a ^ t!c2 = u (i e- 3M/cpa ', .... (57) 



since «!= V 3H when H is small. 



Now the reading of a thermometer whose temperature is 

 changing depends upon the mean value of the surface con- 

 ductivity, h, over its surface. If the mean value of h varies 

 when the thermometer is oriented differently in the wind, 

 the mean temperature must also vary in consequence. Let 

 us consider only metallic thermometers. For these, if 

 K = ch/K is small, we have for the cooling of the thermo- 

 meter from a uniform initial value u : 



(58) 



u being the mean temperature ; and X is proportional to the 

 surface conductivity, h, and is otherwise constant for a. given 

 thermometer. The time, T, required for the mean tempera- 

 ture to drop from u to u /p is given by 



T=l/\Aog e p (59) 



When comparing the cooling of a thermometer in two 

 orientations in the wind, for a given value of T, the values 

 of A are proportional to the corresponding values of log e p, 

 i. e. for the same time of cooling, 



^ = !°^ ...... (60) 



where the suffixes refer to the two orientations in the wind. 

 The following experiment was carried out to determine 

 the ratio liipi 2 for the Short & Mason bimetallic thermometer 

 described in para. 6. The strip in this case was "5 cm. wide 

 and made 4 \ turns with an outer radius of 3*0 cm. The gap 

 between successive turns of the spiral varied from '05 to 

 •10 cm. The surface was silver-plated to prevent rust,. 



