Lags of Thermometers. 67 



V = 2 m.p.h. and V — 10 m.p.h. The values of the lags and 

 displacements obtained will then be upper limits. 



The following data are given for the Kew Recording 

 Thermometer (Mercury cylinder) : — 



K='0197, a 2 = '0137. 

 c = '4:25 cm. = radius of bulb. 

 Z= 10 cm. = length of bulb. 



Case I. V = 2 m.p.h. = velocity of air. 



h = '000103 = surface conductivity. 



H = -002221. 



The formula for the steady lag becomes L = 933G. 

 If Gr^G^ -000306, L = -29° C. 



If G=G 2 = -0000389, L = -04° C. 



These are, respectively, the greatest lag and the lag at the 

 temperature maximum. The value of the displacement of 

 the temperature maximum given by (53) is found to be 



8=10-47% 



Case II. V = 10 m.p.h., 7i = -000515, H = -01111. The 

 values of the lags and of 8 are 1/5 th those in Case I. 



IfG = G l5 L = -06°C. 

 IfG=G 2 , L=-01°C. 



o=2 m 9 s . 



As previously stated, the true value of L and $ may be less 

 than those given here, but they cannot exceed them. 



8. Cooling of Thermometers. Surface Conductivity and 

 Orientation of Thermometer in Wind. 



The formula for the cooling of a thermometer bulb is much 

 simplified when the first term of the series alone need be 

 retained. In the case of a cylindrical bulb, we have for the 

 mean temperature when the initial temperature (at t = 0) has 

 a constant value u , and the air temperature is zero every- 

 where, the effects of the ends of the cylinder being 

 neglected : 



JiAW+H 2 )' • • • • {ai) 



-where /3 n is defined by equation (9). When the first term 



F 2 



