﻿EQUIPMENT AND WORK OF AN AERO-PHYSICAL OBSERVATORY. 17 



And thus, as the authors point out, with the exception of the vapor 

 group 5.5 mm., the potential curve runs inversely with the vapor pressure, 

 and the observed values and the estimated values agree fairly well up to 

 a certj9.in point. When the number of grammes of moisture in a cubic 

 metre of air at the earth's surface exceeds 8, however, the agreement no 

 longer holds. (See diagram, page 18.) 



The aim of all this, as we shall see in discussing Exner's observations, 

 is the determination of a relation between the potential gradient and the 

 humidity and the construction of apparatus in the nature of an electro- 

 hygroscope. One advantage this new method might possess over spectro- 

 scopic methods is at times of cloud formation. Cloudy condensation we 

 know limits rain-band investigation, but the electro-hygroscope would 

 in all probability be serviceable with visible as well as invisible vapor. 

 Indeed, it is not certain but that the potential values would be more 

 strongly marked at such times. 



We must give now as briefly as possible the results of the observations 

 of Elster and Geitel on the Hoher Sonnblick, some 10,168 feet above sea 

 level, and at its low-level station, Kolm-Saigurn. As we said above, such 

 observations, repeated at Pike's Peak and other high-level stations in the 

 United States, would certainly widen the horizon in aero-physical work. 

 Harvard College Observatory has published in its "Annals" the meteoro- 

 logical observations made at Pike's Peak for a given number of years. 

 If one will turn to pages 459, 460, and on, he will find such observations 

 as the following : 



"... faint auroral streamers and beneath them the usual sheet 

 lightning flashed incessantly. ... " 



"... at night the summit capped by a cloud so small that the 

 observer at the base could hardly see it, and was frequently lit up by 

 flashes of lightning. ..." 



"... heavy snow, with thunder and lightning. ..." 



"... electricity increased and decreased with the fall of hail — 

 a fact noticed in all hail-storms at the station. ..." 



In few ways, therefore, do we think that investigation under the con- 

 ditions of the Hodgkins bequest could be more efl'ectively undertaken 

 than in investigation of the electrical condition of the atmosphere at 

 some such station as Pike's Peak. 



The Sonnblick observers found — 



1. " The intensity of the most refrangible rays of the solar spectrum, as 

 measured by the discharging action on negatively electrified surfaces of 

 amalgamated zinc, increases with the height above ground in such a 

 manner that at a height of 3,100 metres it is twice as great as on ordi- 

 nary level ground." 



