AT KEW OBSERVATOEY, 1898 TO 1912. 139 



Data for r.Jt\ were available for 48 individual months. The value was TOO in 

 4 cases ; in 32 cases it was less, and in 12 cases it was greater than unity. Combining 

 the months of the same name from different years, the means for the 12 months 

 January to December in order were 



I'Ol, 0'96, 0'95, 1'02, I'Ol, 0'99, TOO, TOO, (T99, 1'02, 0'95, 0'91, 



giving as arithmetic mean 0'98 4 . 



The observed departure of r.Ji\ from unity is in the direction indicated by theory 

 and is of the right order, but is in excess of the calculated difference. A close agree- 

 ment could hardly be expected on account of the experimental difficulties, and there 

 is the further important fact that the potential gradient observations refer to the 

 forenoon a little after 10 a.m., while the ionic observations refer to about 3 p.m., and the 

 diurnal variation in the ionic charges has not been ascertained. 



On examining details, however, it will be recognised that some other factor 

 probably comes in. While the mean value of p for the whole 12 months is+52xlO~ 9 , 

 its mean value for the 6 summer months, April to September, is + 63 x ICT", and that 

 for the 6 winter months only + 40 x 10~ 9 . 



Thus the decline per metre in the potential gradients in summer and winter should 

 be nearly in the ratio of 3 : 2. The absolute potentials in these seasons, however, as 

 will be seen presently, stand to one another roughly in the ratio of 2 : 3. Thus the 

 departure of rjr v from unity should on the average be fully twice as large in summer 

 as in winter. 



The monthly means recorded above, however, give I'OO for the summer value of 

 r J r \> as compared with 0'97 for winter. The results for r.J-t\ during the first year of 

 observation, 1910, fluctuated rather markedly, values in excess of unity being 

 especially frequent in the first few months, when experience was at a minimum. If 

 we omit all data for 1910, the summer value of r.Jr\ falls to 0'99, the winter value 

 remaining 0'97, and the mean for the whole year becomes 0'98 . 



As before, the summer value of r 2 />\ exceeds the winter one. Assuming the 

 difference to be real, a probable explanation immediately suggests itself. The extent 

 of level turf in the Observatory garden is limited. It is immediately surrounded by 

 ground devoted to vegetables, and this in turn is enclosed by a hedge about 4|- feet 

 high. The vegetation must inevitably have some slight effect in lowering the 

 potential at small heights, and this effect will naturally be greatest at the season when 

 vegetation is most exuberant. Also while the grass at the place of observation is 

 mown more frequently in summer than in winter, its average height is probably 

 greater at the former season. These disturbing influences will naturally be felt 

 mainly, if not entirely, by the potential observations at the 1 -metre level. 



6. Before concluding his engagement at the Observatory, Mr. DINES took some 

 observations intended to determine the relation between the potential gradients 

 obtained in the old way with the old apparatus, and those obtained in the new way. 



T 2 



