41 ON THE RECENT SECULAR PERIOD 



the phenomenon, therefore, appears to me to remove it from the sphere of terrestrial 

 causes, and leads us to look to the nebulous matter, which is known from the 

 zodiacal light, and from meteoric showers, to exist in the planetary spaces of 

 dimensions sufficiently ample to correspond to the extent of these exhibitions. 

 There is, however, much reason to believe that the nebulous body which affords 

 the auroral matter does not, in fact, cover the entire extent of display from east to 

 west, but that the diurnal revolution of the earth brings successive portions of the 

 earth's surface under that body, or through the portions of it which afford the 

 auroral matter. This view of the origin of the aurora is entirely consistent with 

 the fact that its exhibitions take place within the atmosphere. The material comes 

 from the planetary spaces, but is visible only when it traverses the atmosphere. 



Secondly, we argue the foreign origin of the aurora borealis, from the fact that, 

 in places differing many degrees in longitude, the different stages of the aurora {the 

 beginning, maximum, and end) occur at the same hour of the night. When a great 

 exhibition, like that of November 17, 1848, is seen at various places from east to 

 west, as in London and in New York on the same evening, on comparing the hours 

 at which corresponding parts of the display took place, we find the hour of the 

 day the same at both points. At both it began (say) at an hour after sunset, at 

 both it came to its maximum between 10 and 11 o'clock, and ended at both at 3 

 o'clock. Such a correspondence, at given hours of the night, we have traced in 

 numerous examples, and it will generally be found to hold good. But were the 

 display produced by any cause acting simultaneously over the regions where it 

 prevailed, then these corresponding parts of the display would occur at different 

 hours of the night, corresponding to the longitude. If it began at New York at 

 7 o'clock, it would begin at London at 12. If it arrived at its maximum at 10 

 o'clock at New York, at San Francisco the time of maximum would be half past 

 5. But the maximum was nearly at the same hour at both places. Were the 

 cause of a terrestrial nature, whether exhalations from the earth, or precipitations 

 from the atmosphere, or electricity flashing from the denser portions of the atmo- 

 sphere towards the rarer, or the circulation of currents from the upper terminations 

 of electrical atmospheric columns from the equator to the poles, after the manner 

 of De La Rive ; or from one point to another through the rarefied air of the upper 

 regions, after the manner of Priestley, Morgan, and the elder electricians generally; 

 or were the cause the flashing of electricity along inclined columns of metallic 

 vapor, standing over the whole region through which the aurora prevails, after the 

 manner of Biot; in each and all these cases, corresponding stages in the exhibition, 

 such as the beginning or the maximum, could not happen at the same hour of the 

 night unless the cause operated from east to west at the same rate as that at which 

 the earth turns on its axis; and this would be required, not only in a single 

 instance, but in nearly every instance wherever the phenomenon has been observed. 

 Indeed, the progress of the supposed agent westward, would have to vary in 

 different latitudes, corresponding to the diurnal velocity due to the latitude. But 

 if, as we suppose, the source of the aurora is extrinsic to the earth, consisting of 

 portions of a nebulous body attracted down to the earth from the part of the body 

 to which the earth successively approximates nearest, then as places differing in 



