Observations made with a Black Bulb Thermometer. 93 



upon the temperature differences, which should therefore be the 

 same for all the stations and for every month of the year. But the 

 air having some absorbing power the differences should be greatest 

 when there is least air for the sun's rays to pass through ; that is to 

 say, at the highest stations and in the summer months. Up to 

 Chakrata the excess temperature of the solar thermometer does 

 increase with a fair degree of regularity, but it appears to be less 

 at Leh than at Chakrata, contrary to all theory. There is also no 

 regular increase apparent in the heating power of the sun as the 

 season changes from winter to summer. The truth is that the 

 indications of the black bulb thermometer are affected by so 

 many disturbing causes that after all possible corrections they 

 are of little or no value for inter-comparison ; though with the 

 same thermometer at the same place and under absolutely 

 constant conditions of exposure the figures for one year may 

 be to some extent comparable with those of another." The 

 probable error in this criticism is, of course, that any sort of 

 air is an absorber of solar radiation — its quantity, not its quality, 

 determining the amount of the absorption. Be that as it may, 

 I have reduced the differences of maxima (which Hill calls the 

 "excess temperature of the solar thermometer") to a common 

 standard of cloud, relative humidity, and dew-point by means of 

 the results obtained for Kenilworth in Tables III., V., VI. The 

 steps of the process will be followed in the last three columns of 

 Table VIII. Considering the outstanding amount of uncertainty 

 in the different elements the agreement of one station with another 

 shown by the last column is remarkable. Chakrata is the only 

 station differing materially from the others, and this arises probably 

 because faulty observing has given too great a temperature excess 

 to start with. Adelaide, as it happens, is in nearly the same latitude 

 as Leh ; Kimberley is some 7° nearer the equator, and so should be 

 expected to show a somewhat greater difference of maxima between 

 sun and shade than either of these two. 



I have chosen Himalayan stations for this comparison because 

 they stand at different altitudes near the same latitude. The 

 outcome goes, I think, to show that the observations made in 

 various places may be much more readily comparable than has 

 sometimes been supposed. Given a uniform system of observing, 

 under definite conditions of exposure, then it seems a fair inference 

 that valuable and comparable data can be obtained. It is to be 

 hoped, at any rate, that the hard names the black bulb thermometer 

 in vacuo has been called will not deter observers from continuing to 

 use it to the very best of their ability. 



