490 Dr. C. Chree : Results obtained at Kew Observatory with 



means the case at Kew, more especially in the afternoon, 

 when haze is a frequent accompaniment of sunshine. 



§ 12. It was originally intended to treat relative humidity 

 in the same way as temperature,, pressure, and cloud, and this 

 was actually done for one whole year. The mean electric 

 results, however, from the groups of days of high and of low 

 relative humidity were so closely alike for all the seasons that 

 it appeared unnecessary to proceed further. 



§ 13. The last meteorological element to be considered in 

 detail was wind direction. In some months one direction 

 was so dominant that grouping of days presented difficulties. 

 Supposing the wind to be westerly on 9 days out of 11, little 

 significance can be assigned to the numerical size of the 

 difference between means derived from the 9 days and from 

 the 11. Accident is likely to play too large a part. The 

 method actually adopted was as follows : the days of each 

 month were grouped under the four fundamental directions, 

 N, E, S, and W. If the wind were N.W., or N.N.W., or 

 W.N.W., it was grouped under both N and W. The values 

 of a + , a_, q and P were then found for each of the four 

 directions — or for all that were represented — for each month. 

 Taking for example a + , the direction giving the largest mean 

 value was regarded as taking the first place in a competition, 

 the direction giving the next largest mean value the second 

 place, and so on. Out of the whole 36 months there were 

 19 on which the largest mean value of a + was associated with 

 a South wind, as compared with 10, 4, and 3 respectively on 

 which the largest value was associated with a West, a North, 

 and an East wind. The second place on the list was taken 

 7 times by a South, 19 times by a West, 6 times by a North, 

 and 4 times by an East wind. These results form the first 

 two rows in Table XL under a + . The data for a_, q, and P 

 possess the same significance. 



The sum of the figures for " first " and " second " always 

 totals up to 36, but the same is not true of the figures for 

 "third "and "fourth" because in some months not more 

 than 3 or even than 2 wind directions were represented. 



A glance at Table XL shows a very pronounced association 

 of high values of a + and a_ with southerly and westerly 

 winds, and an equally pronounced association of high values 

 of P with northerly and easterly winds, q seems to be inde- 

 pendent of the wind direction. 



The influence of wind direction on a + , a_, and P is much 

 less marked in winter than in the other seasons. Winter 

 contained 5 of the 7 months in which a + was highest with a 

 N. or E. wind, 7 of the 9 months in which a_ was highest 



