&0 On Some Observations of Atmospheric Electricity. 



widely different potentials of the air the same height from 

 the ground in two different places but little removed from 

 one another will be constantly found ; and even in the most 

 serene days, when no clouds are seen, no disturbance appa- 

 rent, sudden and inexplicable variations sometimes occur. 



The passing of clouds constantly alters the electric condi- 

 tion of the air on the earth's surface ; and indeed all the 

 induction and other phenomena which one can exhibit at 

 the lecture table with an electric machine are in almost 

 incessant operation in the earth's atmospheric envelope. In 

 observing the electric condition of the air we adopt Sir 

 William Thompson's method, and select a certain stratum of 

 air, say six or eight feet from the ground and four to six 

 feet from the walls of any building or other object projecting 

 above the surface of the ground, and the collecting point is 

 always maintained in this position ; the measurement given 

 by the apparatus being the difference of potential between 

 the surface of the earth and the air at the selected point. 

 If the air is at the same potential as the earth the instru- 

 ment will indicate zero, if it be at a higher potential it will 

 indicate above zero, and below if at a lower ; the latter 

 state of things may be considered as abnormal. The unit 

 of measurement adopted is the difference of potential 

 between the two poles of a galvanic battery cell, so that 

 the statement that the electric potential of the air at six 

 feet above the ground was equal to 300 Daniell's elements 

 means that the difference of potentials between the air and 

 the surface of the ground was equivalent to that between 

 the two poles of a Daniell's battery composed of 300 cells. 



The photographic curves obtained with our electrometer 

 have not yet been tabulated, but some facts have already 

 been deduced, of which the following perhaps are the most 

 interesting : — 



In calm and serene weather a regular diurnal maximum 

 and minimum are very marked, the highest part of the 

 curve taking place about 7 a.m. and the lowest about 2 p.m. 

 A second maximum about 9.30 p.m., and a second minimum 

 about 1 a.m., are also indicated". 



Hot winds are always accompanied by strong negative 

 tension, and more especially so if dust is present in the air, 

 when sparks can often be got from the collector. The usual 

 turning of the wind from north to south-west is always 

 accompanied for a short period by a high positive tension. 

 In squaUy weather, rapid and large variations from low nega- 



