218 Observations upon Auroral and Optical Phenomena. 



expansions and variations. This lower limiting line was not exactly 

 horizontal, — being elevated at first about 3° more at its western than 

 at its eastern extremity. As the cloud moved round westward, this 

 line assumed gradually a position more nearly horizontal. A re- 

 markable circumstance was that the cloud would disappear at short 

 intervals; after which it would gradually but rapidly light up again 

 in the same place, and with the same shape nearly as before. This 

 process of alternate vanishing and reappearing was continued at in- 

 tervals of from half a minute to two or three minutes and more, dur- 

 ing all my observations. The cloud had not only moved round to 

 the west, but had increased, towards the last, in its horizontal extent 

 to about 33° ; but its vertical breadth was not materially changed, 

 and the lower horizontal limit retained its well defined and rectilin- 

 ear character. It disappeared rather suddenly at about a quarter 

 before ten, — the altitude of its lower limit being then 19^°. 



On arriving at New Haven, a day or two afterwards, I found that 

 the same isolated appearance had been viewed attentively by sev- 

 eral persons, — by Mr. Stanley, now Professor of Mathematics in 

 Yale College,^— by Mr. E. C. Herrick, and by an intelligent gentle- 

 man, a member of Yale College. From each of these gentlemen I 

 obtained, previously to communicating my own observations to them, 

 a careful description and delineation of this phenomenon — its changes 

 and its location among the stars, — for the purpose of making three 

 independent comparisons with my own particular minutes respecting 

 the same. By a comparison of the aspects, the changes, the mo- 

 tions, and the time of disappearance, there could not remain a doubt 

 that this cloud, seen at New Haven, was identical with the cloud 

 seen by me at New Britain. In this comparison four circumstances 

 conspired to favor a correct result. 



First. There was no other auroral appearance seen in any part 

 of the heavens. Therefore the different observers were viewing the 

 same thing; which removes one of the usual and most perplexing 

 uncertainties attendant on such comparisons. 



Second. The two stations, twenty six miles apart, were nearly in 

 a line with the cloud, and for a part of the time exactly so. 



Third. The lower limit of the cloud, being a well defined line, 

 and continuing so throughout, oiFered an unusually definite object for 

 the estimation of altitudes, and for location by the stars. 



Fourth. The motions being chiefly horizontal, and the changes of 

 ^Mtiide very slow, an error of several minutes as to the time would 



