﻿of the Atmosphere and Surface of the Earth. 853 



possessed by the atmosphere is of course positive, and in 

 Bloemfontein, at an elevation of 4500 it., seldom exceeds a 

 maximum value of 200 volts per metre. But during a dust 

 storm the sign of the charge changes, and gives rise to 

 values exceeding 500 volts per metre. It is not necessary 

 for the instrument used for measuring the potential to be 

 actually in the dust itself, for a " dust devil," i.e., a whirl- 

 wind which carries up a column of fine sand, sometimes two 

 or three hundred feet in height, can produce a very per- 

 ceptible effect at a distance of more than two miles, reversing 

 the positive charge in the air and giving a large negative 

 one. 



It is generally believed that, as a consequence of the air- 

 earth current flowing into the earth, the latter possesses a 

 negative charge ; an explanation of the negative potential 

 caused by the dust storm appeared to lie in assuming that 

 the particles of sand carried up by the wind, being them- 

 selves negatively charged, gave rise to the negative potential. 

 But this is not the case, for, as will be shown, the dust 

 particles are positively charged, and, at all events in the 

 situation where the observations were carried out, the surface 

 of the earth is also charged with positive electricity. 



The climate of South Africa is, on account of its dryness, 

 eminently suitable for observations on atmospheric elec- 

 tricity ; but it is quite possible that similar conditions may 

 prevail elsewhere, and the following experiments may perhaps 

 be successful in other places. 



(1) The charge upon the Earth's Surface. — If the earth is 

 a charged body, it seemed that it might be possible to remove 

 a portion of the charge, and test its magnitude and nature by 

 an electroscope. So with this object the writer used a large 

 proof plane, about 500 sq. cm. in area, composed of gilded 

 wood (originally made by Sir Wm. Snow Harris for his 

 weight electrometer) attached to a long ebonite rod ; the 

 surface of the earth and objects upon it were tested by 

 touching for an instant, and then carrying the proof plane 

 to an electroscope placed upon a step-ladder at a height of 

 2 metres above the ground. When this was done the 

 proof pit ne was found to have a positive charge in all cases. 

 The magnitude of the charge, however, depended upon the 

 nature of the material with which the proof plane had been 

 in contact, the mo.^t satisfactory surface being that of the 

 firmly compacted soil. 



The method of conducting the experiment was then modi- 

 fied, so that the observer could take observations without 

 leaving the electroscope. 



