100 



Dr. C. Chree. Observations on Atmospheric 



have the influence of a massive stone pillar some 2J square feet in 

 section. A calculation of the potential gradient which regards the 

 observations at A as referring to a spot 60 inches above the ground 

 in the open is certain to give an under- estimate. As it is impossible, 

 however, to dispense with a support of some kind, and the presence 

 of the observer is also a disturbing influence, no exact allowance can 

 be made for this. 



There have been four principal series of observations. In the first, 

 occupying part of November and December, 1894, observations were 

 taken, when practicable, once a day at stations A, B, C, D, and latterly 

 at E also. In the second series, during part of March and April, 

 1895, observations were usually taken about 10.30 A.M. and 4.30 P.M. 

 at each of the stations except F. The third series, during part of 

 June and July, 1895, closely resembled the second; and the only 

 material difference in the fourth was the substitution of station F for 

 station D. 



No observations were taken on Sundays or on Saturday afternoons. 

 The observations were taken in a fixed order, and, thanks to the skill 

 of the observer, Mr. E. G. Constable, a complete set of readings 

 occupied only some seven or eight minutes. The time scale of the 

 electrograph curves is far from open, and for this and other reasons I 

 have judged it best not to attempt to reduce the readings with the 

 portable electrometer to a common instant. 



Comparison of Results at the different Stations. 



5. I have taken A as base station, and have found the ratios 

 borne to the individual readings there by the corresponding readings 

 at the other stations. 



Let r A , r E represent corresponding readings at A and B, and let 



_ 1 



n 



B/A 



where 2 denotes summation for a series of n observations. Then r 

 maybe called the mean value of the ratio for the series of observations. 

 Also let us apply the term percentage deviation of the ratio from its 

 mean to the quantity 



X100, 



B/A 



in which the terms in the numerator are taken irrespective of sign. 



Table II gives the extreme and mean values of the ratios during 

 each series of observations, excluding three or four occasions when 

 negative potentials were met with. 



