66 Mr. A. R. McLeod on the 



following values for the cylindrical bulbs considered, and 



the values of 6H ^-={^ + H(H-l)} 



for the spherical bulbs considered in para. 4 : — Mercury 

 sphere, *999 ; alcohol sphere, 'S65 (3 terms given); Mer- 

 cury cylinder, 1*000 ; alcohol cylinder, '978. 



If H is small we have very nearly j3 1 = \/2H, and so we 

 find from (50) 



'-toM-gt) < 51 > 



But when H is very small, the value of: the steady lag after 

 the inversion is passed is 



GV 2 



Hence 



L\ /G'-G 



S =G 



^(V) ™ 



Note that 8 depends only on the ratio of G to G', and not 

 on the absolute values of the gradients. In fact the value 

 of 8 as given by (51) is 



For the mercury cylinder considered in para. 4, we 

 found H = *0667, and so (53) should apply. Calculation 

 yields the same values for 8 as were found in para. 4. 



As a numerical example we may consider the Kevv 

 Recording Thermometer. To obtain the values of the 

 temperature time gradients, a series of monthly mean, 

 diurnal temperature readings (Kew Records) was examined. 

 The maximum value of the gradient occurs in the mornings 

 (March-Sept.) and is 2 o, F. per hour, or in degrees centi- 

 grade and seconds, G 2 = "000306. The mean gradient for an 

 hour preceding or following the maximum was found to be 

 •25° F./hour or G 2 = *0000389. The surface conductivity is 

 really given by a relation of the form 



A = /, + -0000515 V, 



where the constant term A is negligible for aeroplane speeds, 

 so that equation (30) then holds. In general, therefore, it 

 will not be permissible to use (30) with small values of V. 

 But as an example we shall take the cases in which 



