READINGS OF A BLACK BULB THERMOMETER. 



521 



cannot, for instance, be said that when the Black Bulb is 40° above shade temperature, 

 radiation is double what it is with a difference of 20° ; all we know is that it is greater, 

 but not how much greater. 



The times when the shade temperature passes its mean value are, as usual at most 

 stations, about 9 a.m. and 9 p.m. ; that is, from 9 a.m. to 9 p.m. temperature is above 

 the mean of the whole day in any month, and from 9 p.m. till 9 a.m. below it. But the 

 Black Bulb readings at these hours are very different : at 9 a.m. the difference from the 

 shade temperature is approaching its maximum — in June and July, indeed, it is nine- 

 tenths of the greatest difference in the whole day ; while at 9 p.m. direct solar influence 

 on the Black Bulb has practically vanished, and it reads about the same as the Shade 



Hour. 





Shade Temperature 





Excess 



of Black Bulb 



ABOVE 



Shade Temperature. 



T 



wo Years' Means, 1890- 



91. 



Two Yeai 



•s' Means, 1890-91. 



June 



1891. 



G.M.T. 



June. 



July. Aug. 



Sept. 



Mean. 



June. July. 



Aug. 



Sept. 



Mean. 



Three 



fine days. 



10, 11, 12. 



Three 

 foggy days. 

 13, 14, 15. 



4 A.M. 



5 „ 



6 „ 



38-2 

 38-3 

 38-6 



37-3 

 37-2 

 37-4 37-4 



39 ; 2 



38-1 



2-4 07 



9-5 4-2 



18-5 11-0 



3 V 8 



i-4 



87 



10-0 

 33-9 

 51-9 



0-9 



4-1 



10-3 



7 ,. 



8 „ 



9 „ 



38-8 

 39-2 

 39-7 



377 37-6 

 38-2 38-0 

 387 38-5 



39-2 

 39-6 

 40-1 



38-3 



38-8 

 39-2 



26-2 167 

 3T2 22-3 

 367 27-3 



10-0 

 15-8 

 21-0 



7-0 

 11-2 

 17-2 



14-9 

 20-1 

 25-6 



63-0 



71-5 

 75-9 



18-3 

 17-4 

 31-6 



10 ., 



11 „ 

 Noon 



40-2 

 40-6 

 41-1 



39-2 39-2 

 397 39-8 

 40-2 40-2 



40-6 

 41-4 

 41-8 



39-8 



40-4 

 40-8 



40-2 27-6 

 41-2 30-0 

 38-9 28-9 



24-0 

 26-2 

 26-1 



20-9 

 24-4 

 22-8 



28-2 

 30-4 

 29-2 



79-3 



80-3 

 807 



24-6 

 21-0 

 22-6 



1 r.M. 



2 „ 



3 „ 



41-4 

 41-6 

 41-4 



40-5 40-5 

 40-6 40-6 

 41-0 40-8 



42-0 

 42-2 

 42-2 



41-1 

 41-2 

 41-4 



34-8 27-5 

 34-8 29'2 

 31-8 24-9 



257 

 22-4 

 18-4 



24-0 



21-8 

 18-8 



28-0 

 27-0 

 23-5 



80-8 

 793 



75-8 



16-6 

 21-6 

 257 



4 ,, 



5 ,, 



6 „ 



41-6 

 41'2 

 40-6 



40-7 40-4 

 40-5 40-1 

 40-0 39-8 



42-0 

 41-6 

 41-1 



41-2 

 40-8 

 40-4 



31-4 20-3 

 24-0 16-4 

 18-0 11-8 



15-7 

 107 



7-4 



14-0 



10-6 



4-3 



20-4 

 15-4 



10-4 



70-3 

 61-5 

 50-5 



15-5 

 9-7 

 67 



7 „ 

 9 ,. 



40-3 

 39-8 

 39-4 



39-5 39-3 



39-0 



387 







12-8 7-2 

 3-5 2-4 

 0-0 0-3 



i 



2-8 







30-8 

 6-2 



4-2 

 2 3 



thermometer. This is, of course, quite in harmony with the indications of the latter — 

 at 9 a.m. solar radiation is in excess, and the air is therefore getting heated ; at 9 p.m. 

 terrestrial radiation is in excess, and the air is cooling. In theory, the time of maximum 

 shade temperature should be when solar radiation has passed its maximum, and diminished 

 so far that terrestrial radiation just balances it, and the air is neither heated nor cooled. 

 The table shows that the maximum shade temperature occurs about 3 p.m., at which 

 hour the excess of the Black Bulb is greater thau it is at 8 a.m. and less than at 9 a.m. 

 If, therefore, the reading of the Black Bulb at 3 p.m. represents a solar radiation equal 

 to terrestrial, we would expect a similar equality about 8 or 9 a.m. Strictly, if this 

 equality exists, this latter — 8 or 9 a.m. — ought to be the time of minimum shade 

 temperature. It is not so, but a glance at the summer temperatures will show that 



ROY. SOC. TRANS. EDIN. -VOL. XLII. 3 X 



