and Nature of the Cosmic Electric Rays. 49 



my intention in this paper' to state briefly some of our 

 ■principal results, and to see what conclusions may be drawn 

 from them with regard to the question about the physical 

 properties of the solar electric rays. 



§2. Results of Height Measurements. 



The average height of the lower boundary is given in 

 Table I. for the most conspicuous auroral forms : — 



Table I. 



-r, Number of points tt . ,, 7 



Form. -, , • L j Height. a. 



determined. & 



km. 



Kays : 61 11316 +5-22 



Draperies 409 109-75 +T81 



Drapery-shaped Arcs ... 888 106-58 -1-36 



Split Bands ~i 



* [ 409 109-08 +1-14 



Diffuse Arcs J 



Pulsating Aurora 160 105 97 -197 



Total for all forms ... 1927 107*94 



d is the difference from the mean volume. 



We see the remarkable fact that the average value is very 

 nearly equal for all forms. The highest value is — strangely 

 enough — found for the ray-form, contrary to the impression 

 one gets by direct examination of an auroral display. It 

 -often appears as if the rays were shooting down far below 

 -the other forms. In the case of rays and draperies there 

 should be no systematic error which should tend to make the 

 heights greater than those of the other forms. Possible 

 systematic errors should have the opposite effect. Thus our 

 results shoiv that the draperies and ray-forms do not on an 

 ■ average come so near to the ground as do the other forms. As 

 we shall see later on, this result is of great importance in 

 connexion with our studies of the distribution of luminosity 

 along the ray streamers. 



The average heights of the upper limit of the various forms 

 are given in Table II. 



Table II. 



Form. Number of points. Height. 



km. 



Kays 55 229*25 



Draperies 215 17625 



Drapery-shaped Arcs 174 174*83 



Diffuse Arcs 57 L48'40 



Phil Mag. S. 6. Vol. 42. No. 247. July 1921. E 



