Page:Proceedings of the Royal Society of London Vol 60.djvu/119

104 By “ sunshine in hours ” is meant the number of hours of sunshine measured by the Campbell-Stokes recorder up to the time of observation. The data under this head have been limited to the most sunny series of observations, viz., II and III.

The results are exhibited in Table VI, which shows also the maxima and minima values of the meteorological elements observed during the several sets of n observations.

There is in Table VI no uniform and conspicuous connexion between the value of rE/A, or rF/A, and the corresponding value of any one of the meteorological elements considered. In the case alike of barometric pressure and temperature the second mean—answering to the nlowest values of rE/A or rF/A—is higher than the first in five instances out of six. The differences between the two means are generally, howevei’, so small that the phenomenon may be purely accidental. Ip the afternoon observations of series II there is a somewhat conspicuous association of a low value in rE/A with a high value of previous sunshine ; but in series III there is no trace of such a phenomenon.

The question whether there may not be certain occasional types of weather, whose influence is marked in such a table as VI, which are associated with either a high or a low value of the ratio r£/A, remains, I think, open. Evidence is in my hands which leads me to believe that during a low ground fog the potential gradient as a rule is decidedly higher near the ground where the fog is thick than higher up where the fog is slight.

§ 9. The conclusion I am disposed to draw, though I regard it as only a probability, is that such general phenomena as diurnal or annual variation of potential near the ground in the open may be deduced with fair accuracy by applying a constant factor to the records of a portable electrometer, employed regularly at a fixed point on the Observatory roof or near its walls. It must be remembered, however, that all six stations were comparatively close together, and that the equipotential surface passing through the highest station would be in the open perhaps only 14 or 15 feet above the ground. There is thus no evidence to warrant the deduction of conclusions for a spot a mile or two away or a few hundred feet above the ground.

On the trustworthiness of individual results deduced by means of a constant factor, one would not, after inspecting Tables II and III, be disposed to place much reliance. This question can hardly, how-' ever, be settled satisfactorily unless one have apparatus for taking the observations at the different stations absolutely simultaneously. The largest departures from the means in Tables II and III are