1898.] ABBE — THE ALTITUDE OF THE AURORA. 5 



lated the altitudes of specific beams and arches by trigonometrical 

 or equivalent methods have generally found figures indicating alti- 

 tudes between twenty and a hundred miles. Perhaps the highest 

 altitudes that have been deduced were the following : Dalton, 150 

 miles; Loomis, 400 to 600; Bergman, 468; Boscovich, 825; 

 Fournerius, 1006; Twining, iioo ; Boiler, 2000 kilometers, or 

 1243 miles. 



Those who delight in numerical calculations accept these larger 

 altitudes and content themselves with saying that the altitude of the 

 aurora ranges from 50 miles upward to 1000. The experimental 

 physicists, by studying the analogies between the auroral light 

 and the discharge of electricity through vacuum tubes, have shown 

 that the auroral phenomena harmonize in part at least with those 

 observed in vacua such as might occur at moderate altitudes. Thus, 

 Miller and De La Rue give altitudes of from ten to forty miles. 

 Espy and Bache maintained that observers a few miles apart did 

 not and could not have observed the same arches. The most care- 

 ful observers have in many cases defended the accuracy of the 

 observations made under circumstances that admit of no doubt that 

 the auroral light in the free atmosphere often emanates from points 

 within a few yards of the observer. 



Lemstrom has sought to reconcile the diverse conclusions by 

 maintaining that while many auroras are quite high up and belong 

 to the upper air, yet those in extreme northern latitudes most 

 generally belong to the lowest strata and follow the unevenness of 

 the ground, appearing as glows around the mountain top, or as rays 

 directed toward prominent objects. 



The object of the present paper is to study some of the numerous 

 observations, calculations and opinions bearing on the nature and 

 the altitude of the auroral light. We shall not especially consider 

 the electrical origin, or the source of the electricity, but simply ac- 

 quiesce in the universal conviction that it really is one form of 

 electrical discharge, our main object being to ascertain whether we 

 can in any way definitely fix its locus in the atmosphere. 



The most instructive method of procedure consists in taking up 

 the consideration of a number of authorities in chronological order, 

 by which means one is led to appreciate the slow progress of knowl- 

 edge and the difficulty which many investigators have felt, from 

 time to time, in giving up preconceived views without having any- 

 thing better to accept in their place. There is nothing more diffi- 



