7 
218 Observations upon Auroral and Optical Phenomena. 
expensions and variations. This lower Jimiting 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 194°. 
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, offered 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 
altitude very slow, an error of several minutes as to the time would 
