﻿14 EQUIPMENT AND WORK OF AN AERO-PHYSICAL OBSERVATORY. 



and the ver}^ recent paper of Kelvin on the Subtraction of Vapor from 

 Air and the Electrification). The potential charted for any given moment 

 upon the synoptic map will give, in the general electrification of the 

 lower air strata, significant equipotential lines and areas. The time is 

 ripe for such a preliminary survey, or, as Sir William Thomson once 

 called it, '' electro-geodesy " — in brief, an extended synchronous survey 

 of the potential of the lower air. 



We may begin our plea for such work by a reference to the work of 

 Professor Franz Exner, of Vienna. With praiseworthy persistency he 

 has determined the potential values in all localities accessible to him, 

 and with some approximation the potential gradients at various eleva- 

 tions. The work of Elster and Geitel, especially their later work, con- 

 stituting what may be termed researches in electrical actinometry, is- 

 a natural outcome of and supplement to Exner's work. As we shall 

 see, Exner's determinations were all made with portable electroscopes 

 or electrometers, and while the, instruments used by him differ in de- 

 sign from those (in our opinion preferable) used in the United States^ 

 Great Britain, and France, the scope and method of work have been 

 practically similar. The differences in results are mainly of degree, 

 Exner having carried his work further. The aim throughout has been 

 the exploration of the electrostatic field of the earth. In the experi- 

 ments of Elster and Geitel at Wolfe nbuttel (see " Sitz. Akad. Wien.," 

 June, 1892, and subsequently; also " Nature," March, 1893), the direc- 

 tion of research has been that of the relation of the potential values to 

 the intensit}^ of ultra-violet radiation. This we see at a glance opens up 

 a new field of investigation in the discovery that ultra-violet light ac- 

 celerates the dissipation of an electrical charge, and there is no telling 

 what further developments may come in both electrometry and acti- 

 nometry. 



S. V. Arrhenius C" Meteor. Zeits.," vol. v, p. 297, and "Phil. Mag.," 

 July, 1889) touches upon the infiuence of solar radiation on the electrical 

 phenomena of the earth's atmosphere, and shows that when the air was 

 irradiated by ultra-violet light it conducted like an electroh^te. We recall 

 that Hertz found in his experiments that the receiver required continual 

 readjustment, either because of the shaking or the slight burning of the 

 points, and that ultra-violet light falling on the " vibrator " prevented 

 its proper action, the sparking in the " resonator " ceasing or becoming 

 feeble. This discharging action seemed to be particularly noticeable 

 when the violet light fell upon negatively electrified points. Elster and 

 Geitel have shown ('' Sitz. d. K. Akad, der Wissen. Wien.," 99 Band, x 

 heft, s. 1011) that a body with a negative charge is discharged under the- 



