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$ 13. It seemed desirable to consider what bearing the special experiments might have on the general facts and theories of atmospheric electricity. In this investigation special attention has been given to the possible influence of aqueous vapour on electrical potential, on account of the important researches of Exner, and of Elster and Geitel.

§ 14. Exner has advanced the view that the potential gradient in the open, dYjdn in his notation, and the density of aqueous vapour s im u lta n e o u sly 'present in the atmosphere, are connected by a formula

dVIdn — constant -f-(H-&g0),

where 7c is apparently a constant, the same at all places and at all seasons of the year. Exner, I believe, limited his observations, and presumably the application of the formula, to days comparatively quiet and free from clouds. To test the formula he arranged his observations in groups, according to the amount of vapour present, and compared the mean vapour density—measured in grams per cubic metre—with the mean potential gradient, measured in volts per metre of height above the ground. In the 4 TVien. Sitz., Bd. 99, p. 618, he gives a table including results from Vienna, Wolfenbuttel, St. Gilgen, and India, in which the vapour densities vary from 17 to 23 5. The table unquestionably shows a diminishing mean potential gradient accompanying an increasing mean vapour density. For values of q0 from 12'4 and upwards, however,—including all the Indian and most of the St. Gilgen observations—the change in dV/ is somewhat small and irregular, .An earlier, and somewhat similar, but less extensive table by Exner will be found on p. 434 of ‘Wien. Sitz.,’ Bd. 96.

For information as to Elster and Geitel’s work I am mainly indebted to a long paper by them in the ‘Wien. Sitz.,’ Bd. 101, p. 703, 1892. During 1888-91 they took an extensive series of observations on quiet days at Wolfenbuttel. If I follow their explanations, they took eye observations some ten times a day with an electrometer, in which flame from a lamp acts as collector, and deduced the mean value of the potential gradient dY/dn for the day. They compare these potential gradients grouped according to the value of the vapour density with Exner’s formula, taken to be

(dY/du) (in volts per m etre) = 1410/(1