104 Dr. C. Chree. Observations on Atmospheric 



By " sunshine in hours " is meant the number of hours of sunshine 

 measured by the Campbell-Stokes recorder up to the time of observa- 

 tion. 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 r E/A , or r F/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 n lowest values of r E/A or r F/A is higher than the first in five 

 instances out of six. The differences between the two means are 

 generally, however, so small that the phenomenon may be purely 

 accidental. In the afternoon observations of series II there is a 

 somewhat conspicuous association of a low value in r E/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 masked in such a table as VI, which are 

 associated with either a high or a low value of the ratio r E/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 rale is 

 decidedly higher near the ground where the fog is thick than higher 

 up where the fog is slight. 



Summary of Results at Different Stations. 



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 remem- 

 bered, 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 





