112 Dr. C. Chree. Observations on Atmospheric 



Method of Treating Kew Observations. 



16. In discussing the Kew observations 1 have in general em- 

 ployed a method differing from the grouping system of Exner and 

 Elster and Geitel, and have also treated the several series separately. 

 It is clear from data mentioned by Exner that the potential gradients 

 for individual members of his groups varied in some instances 

 largely from the mean ; and it was soon obvious that the same 

 phenomenon would present itself if any similar treatment were 

 applied to the Kew results. This is undesirable, because by varying 

 the limits of the groups the accordance of the results with a par- 

 ticular formula may be much improved, or the reverse. However 

 impartially, so to speak, the lines may be drawn, there is undeniably 

 a risk of introducing some fictitious result ; and no critic can feel 

 that he is in a position to judge of the results until he has examined 

 for himself the circumstances of the grouping, a labour he naturally 

 shrinks from. Again a wide range of such an element as vapour 

 density can be obtained at a particular place only by combining 

 results from all seasons of the year. This brings us to a second 

 question. Electrical potential gradient has like vapour density, sun- 

 shine, and temperature, a large annual variation, only, unlike these 

 elements, it is highest in winter. It is thus obvious that when obser- 

 vations from all seasons of the year are treated promiscuously, there 

 is almost sure to be a marked association of high potential with low 

 vapour density, little sunshine, and low temperature ; and a judi- 

 ciously selected formula which makes potential diminish as any one 

 of these elements increases is certain to show some approach to 

 agreement with observation. It is thus desirable to compare together 

 observations from a limited portion of a year, or, even better, from 

 the same season of a series of years. Similar considerations show 

 an advantage in treating separately results from different hours of 

 the day. The isolation of particular seasons and hours has the dis- 

 advantage of reducing the number of observations compared together. 

 This is, however, partly compensated for by the greater homogeneity, 

 of the material. It also enables one in some cases to compare readily 

 the mean potential gradients which answer at different seasons or 

 hours to like values of some one meteorological element (see 23). 



Anticipation of some Criticisms. 



17. The Kew observations were not limited to quiet, compara- 

 tively cloudless days, in the same way as the observations of Exner 

 and Elster and Geitel seem to have been. It may thus not unlikely 

 be supposed that the Kew results are affected by a variety of dis- 

 turbing causes, which diminish their intrinsic value and their suit- 



