236 AURORA BOREALIS. 



4. The colors of the aurora are the same as those of ordinary electricity 

 passed through rarefied air. When a spark is drawn from an ordinary clec- 

 t'-ical machine, in air of the usual density, the light is intense and nearly white. 

 If the electricity be passed through a glass vessel in which the air has been 

 partially rarefied, the light is more diffuse, and inclines to a delicate rosy hue. 

 If the air be still further rarefied, the light becomes very diffuse; it flows readily 

 through a great distance, and its color becomes a deep rose or purple. The 

 same variety of colors is observed during the aurora. The transition from a 

 white or pale straw color to a rosy hue, and finally to a deep red, depends, 

 probably, upon the height above the eartli and upon the amount of condensed 

 vapor presc;ut in the air. 



The emerald green light which is seen in some auroras, is thought to be due 

 to the projection of the yellow light of the aurora upon the blue sky ; for a com- 

 bination of yellow and blue light always produces green. So also during the 

 evening twilight there is frequently a brief period when the western sky exhi- 

 bits a delicate shade of green. This is caused by a combination of the yellow 

 light of the sun with the blue of the celestial vault. If tliis explanation should 

 not seem to account for tlie intensity of the green light which has been noticed 

 in some auroras, the difference may perhaps be ascribed to that well-established 

 physiological principle that when two complementary colors are placed near 

 eacli other, each color appears more brilliant by contrast with its complementary 

 color. 



5. The formation of an auroral corona near the magnetic zenith is the effect 

 of perspective, resulting from a great number of luminous beams all parallel to 

 each other. A collection of beams parallel to the direction of the dipping 

 needle would all appear to converge towards the pole of the needle, as is actu- 

 ally observed; and no other supposition will explain all the appearances. Each 

 observer, therefore, sees the auroral crown in his mngnetic zenith, and it is not 

 the same crown which is seen at different places, any more than it is the same 

 rainbow which is seen by different observers. 



6. The auroral beams are simply illumined spaces, caused by the flow of a 

 stream of electricity through the upper regions of the atmosphere. During the 

 aurora of August 28, 1859, these beams were nearly 500 miles in length, and 

 their lower extremities were elevated about 45 miles above the earth's surface. 

 Their tops inclined towards the south ; the angle with the vertical at New York 

 amounting to 17°. When electricity flows through good conductors, it emits 

 no light. Dry air of the ordinary density is a non-conductor of electricity ; but 

 water is a conductor, and so is rarefied air. When electricity forces its way 

 through dry air of common density, it exhibits a brilliant spark. Through 

 rarefied air electricity passes wiih less resistance to a much greater distance, 

 and with a pale diffuse light. 



It Avas formerly supposed that the electric current necessarily moved in the 

 direction of the axis of the auroral beams; that is, that the electric discharge 

 Avas between the upper regions of the atmosphere and the earth or the lower 

 regions of the atmosphere. But recent discoveries throw some doubt upou this 

 conclusion. When a current of electricity flows through a vessel from which 

 the air is almost wholly exhausted, under certain circuiustauces the light is not 

 uniformly diffused through the vessel, but becomes stratified, exhibiting alter- 

 nately bright and dark bands crossing the electric current at right angles. 

 From this experiment it might be inferred that electricity flowing Jiorizontallij 

 through the upper regie ns of the atmosphere might exhibit alternately brigut 

 and dark bands, having a position nearly vertical like the auroral beams. But 

 this stratification of the electric light is generally ascribed to intcrmiilcnccs in 

 the intensity of the electric discharge, and it does not seem probable that such 

 intermittences could take place in nature with sufficient rapidity to produce a 

 similar effect. It seems, therefore, more probable that auroral beams are the 



