50 ON THE RECENT SECULAR PERIOD 



since. But it docs not appear to me altogether improbable that the zodiacal light 

 is, indeed, the body which affords, at once, the material of the aurora boi'ealis and 

 of meteoric showers. Lying, as it does, nearly in the plane of the earth's orbit, 

 reaching beyond the earth's path around the sun, and having an immense volume; 

 corresponding in constitution, so far as we can judge, with the auroral vapor itself; 

 nothing can be imagined more competent than this to meet all the exigencies of 

 the case. Some portions of it are always near enough to the earth to afford 

 occasional displays of the aurora in its humbler forms, and the known variations 

 of apparent magnitude and brightness to which the zodiacal light is subject, render 

 it competent to meet the most extraordinary exhibitions of the phenomenon. 



In conclusion, we will return to the questions with which we started, and state, 

 briefly, the answers to which we have been conducted. 



1. What is the origin of the auroral vapor, or the material itself which forms the 

 basis of the exhibition? We think it fully proved that the origin of the aurora 

 borealis is cosmical, being derived from the planetary spaces. My meaning is, that 

 the material itself comes into the atmosphere from a region beyond, and so remote 

 as not to be influenced by the earth's rotation, although the phenomena are exhibited 

 in the atmosphere. It must of course be either terrestrial or cosmical. But we 

 argue that the extent of some of the exhibitions is too great to be produced by any 

 terrestrial exhalations or atmospherical precipitations; that in places differing many 

 degrees in longitude, corresponding phases of the exhibition, as the time of arriving 

 at the maximum, for example, occur at the same hour of local time — a fact inconsistent 

 with the doctrine of a simultaneous development of the cause over so vast a space, 

 but entirely compatible with that of a nebulous body in space under or through 

 which successive places on the earth are brought in the diurnal revolution; that the 

 velocity of many of the motions, where there is evidence of actual translation of 

 material, and where the movements are not due to mere flashes or undulations of 

 light, is too great to arise from any terrestrial forces; that the periodicity which 

 attends the phenomenon takes it out of the pale of atmospheric changes and places 

 it within the sphere of astronomical causes, depending on revolutions and the 

 mutual relations of the earth and foreign matter. 



2. What causes the periodicity of these exhibitions ; or why do they occur in certain 

 parts of the twenty-four hours, commencing, for example (in the case of the greatest 

 exhibitions), a little before the end of twilight, and coming to their maximum from 

 10 to 11 o'clock? why in certain months of the year, as in September and November, 

 while they are seldom seen in June and December? and, especially, why do they 

 return in secular periods? 



The occurrence of these exhibitions at certain hours of the night, that is, the 

 diurnal periodicity, a circumstance which belongs to auroras of the polar regions, 

 when it is continual night, as well as to lower latitudes 1 — plainly indicates that the 

 phenomenon has some relation to the position of the sun, although, after much 

 reflection, I have not been able to satisfy myself as to the precise nature of that 

 relation. The most promising chance of solution of the case which has suggested 



See Obs. of Lottin, &c, at Bossekop. 



