294 Prof. Olmsted on the late Periodical Visitation 



leading facts, or laws, and such as are deemed somewhat new, 

 or at least not universally received as established truths. 



1. Time of Occurrence. — An aurora of the first class usually 

 occurs near the end of evening twilight, in the form of a 

 northern light, resembling the dawn ; commonly arrives at 

 its maximum between ten and eleven o'clock, and frequently 

 a little before eleven ; and auroras of the highest order fre- 

 quently continue all night, while those of an ordinary charac- 

 ter commonly end before midnight. 



2. Material of the Aurora. — A great aurora is usually pre- 

 ceded by a large bank, or cloud, of a peculiar vapour, differing 

 in its nature from ordinary clouds, commonly exhibiting a 

 milky appearance, but sometimes a smoky hue, or the two 

 mixed together ; and the extent and density of this auroral 

 vapour resting upon the northern horizon forms the best 

 prognostic we have of the probable intensity of the exhibition 

 which is to follow, composing, as we believe, the material of 

 which the successive forms of the aurora are constituted. 



3. Auroral Waves. — The auroral waves, when peculiarly 

 grand, make their appearance later than the streamers and 

 arches, and usually later than the corona, continued to a later 

 hour of the night, appear at a lower level than the streamers, 

 and roll upwards, in the direction of the streamers, toward 

 the point of general concourse. 



4. Extent. — Auroral exhibitions of the higher order are 

 commonly of a great extent, spreading over no inconsider- 

 able part of the earth's surface, and reaching to a great but 

 variable height. 



5. Periodicity. — Auroras of the first class have three distinct 

 forms of periodicity ; a diurnal periodicity, commencing, 

 arriving at their maximum, and ending at definite hours of 

 the night, as already asserted ; an annual periodicity, rarely 

 or never occurring in June, and the greatest number of the 

 higher order clustering about November, these last bearing 

 a striking resemblance to each other ; and a secular periodi- 

 city, the most remarkable of all, recurring in great series 

 which we have denominated " auroral visitations." The 

 visitations most marked and best defined, occur at intervals 

 of about sixty-five years, reckoning from the middle of one 



