an Elster and Geitel Electrical Dissipation Apparatus. 489 



mist), and the limiting form of extreme clearness distin- 

 guished by the letter v (high visibility) in the meteorological 

 records. Mean values of a + , a_ &c. were got out for the 

 days of bright sunshine in each month, and the algebraical 

 residues remaining after subtracting from these the corre- 

 sponding means from all the observations of the month were 

 totalled for the 36 months. The mean of this sum of differ- 

 ences was finally expressed as a percentage of the mean value 

 of the element for the three years. These percentages for 

 the several elements appear in Table X. The same method 

 was applied in the cases of clear atmosphere and of high 

 visibility. 



Table X. 



Number 

 of days. 



Bright sunshine 

 Clear atmnsphen 

 High visibility 



Percentage excess of element on 

 representative day of special type. 



+ 



144 

 24 



-14 

 + 9 



+ 30 



-15 

 + 12 

 + 21 





 - 1 



-12 



+ 6 

 -10 



To illustrate the interpretation of Table X., take the case 

 of bright sunshine. Out of the whole 411 days of observation 

 there were 125 on which there was bright sunshine during 

 at least the greater part of the dissipation experiment. On 

 the average day of bright sunshine a + was 14 and a_ 15 per 

 cent, below its mean value for the season, while P was 6 per 

 cent, in excess of its mean value. There was no appreciable 

 effect on q. The days of clenr atmosphere include the 24 of 

 high visibility. They exhibit the exact opposite of the 

 phenomena exhibited by the days of bright sunshine, while 

 the days of high visibility are specially conspicuous for the 

 large values of the- dissipation. Whether the apparent 

 reduction of q on days of high visibility may not be in part 

 at least " accidental " is open to some doubt, on account of 

 the comparative fewness of the days. High visibility was 

 mainly confined to summer months, no single example being 

 encountered from November to March. 



The difference apparent between days of bright sunshine 

 and days of clear atmosphere may appear at first sight im- 

 probable, as one is apt to regard the two meteorological 

 conditions as naturally coexistent. This, however, is by no 



