I860.] on Atmospheric Electricity. 285 



the earth's surface, and in the lower strata of the atmosphere; in 

 other words, the total quantity of electricity, reckoned as excess of 

 positive above negative, or of negative above positive, in any large 

 portion of the atmosphere, and on the portion of the earth's surface 

 below it, must be very nearly zero. The quality of non-resistance 

 to^ electric force of the thin interplanetary air being duly considered, 

 we might regard the earth, its atmosphere, and the surrounding 

 medium as constituting respectively the inner coating, the di-electric 

 (as it were glass), and the outer coating of a great Leyden phial, 

 charged negatively ; and even if we were to neglect the consideration 

 of possible deposits of electricity through the body of the di-electric 

 itself, we should arrive at a correct view of the electric indications 

 discoverable at any one time and place of the earth's surface. 

 In fact, any kind of "collector," or plan for collecting electricity 

 from or in virtue of the natural " terrestrial atmospheric electri- 

 city," gives an effect simply proportional to the electrification of the 

 earth's surface then and there. The methods of collecting by fire and 

 water which the speaker exhibited, gave definitively,[in the language of 

 the mathematical theory, the "electric potential*' of the air at the 

 point occupied by the burning end of the match or by the portion of 

 the stream of water where it breaks into drops. If the apparatus is 

 used in an open plane, and care be taken to eliminate all disturbance 

 due to the presence of the electrometer itself and of the observer 

 above the ground, the indicated eifect, if expressed in absolute electro- 

 static measure, and divided by the height of the point tested above the 

 ground, has only to be, (according to an old theorem of Coulomb's, cor- 

 rected by Laplace,) divided by four times the ratio of the circumference 

 of a circle to its diameter, to reduce it to an expression of the number 

 of units in absolute electrostatic measure of the electricity per unit of 

 area of the earth's surface at the time and place. The mathematical 

 theory does away with every difficulty in explaining the various and 

 seemingly irreconcilable views which different writers have expressed, 

 and explanations which different observers have given of the functions 

 of their testing apparatus. In the present state of electric science, the 

 most convenient and generally intelligible way to state the result of an 

 observation of terrestrial atmospheric-electricity, in absolute measure, 

 is in terms of the number of elements of a constant galvanic battery, 

 required to produce the same difference of potentials as exists between 

 the earth and a point in the air at a stated height above an open level 

 plane of ground. Observations with the portable electrometer had 

 given in ordinary fair weather, in the island of Arran, on a flat open 

 sea beach, readings varying from 200 to 400 Daniel's elements, as 

 the difference of potentials between the earth and the match, at a 

 height of 9 feet above it. Hence, the intensity of electric force perpen- 

 dicular to the earth's surface, must have amounted to from 22 to 44 

 Daniel's elements, per foot of air. In fair weather, with breezes from 

 the east or north-east, he had often found from 6 to 10 times the 

 higher of these intensities. 



