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« 

 Prof. Stephen Alexander, of Princeton, N. J., made a verbal 



communication relative to some Miscellaneous Contributions 



to Astronomical Science, which he stated he might present in 



a more permanent form hereafter. 



I. On a Minute Correction in the Position of the Tropic on the 

 Terrestrial Spheroid. 



Prof. Alexander observed, that if the tropic were to be regarded 

 as the parallel of a place, at which the sun was vertical at the time 

 of the solstice, then the central ray from the sun must coincide with 

 the plumb hne of the place, upon the tropic, at which it was then 

 noon. The angle made by this plumb or vertical line, with the 

 plane of the equator, i. e. the measure of the geographic latitude of 

 the place, would exceed the angle with the same plane, made by the 

 line joining the centres of the sun and earth, i. e. the obliquity of the 

 ecliptic, by a very minute quantity. Prof. A. showed how this ex- 

 cess might readily be computed. It amounts to but 0".0217; the 

 measure of which, on the earth's meridian, is rather less than 2 feet 

 2\ inches. 



II. On a Region of Continued Twilight. 



It being considered as established, that the region of twilight ex- 

 tended about 18° beyond the terminator, or boundary of sunshine, it 

 followed, that whenever the sun's declination was less than 18°, the 

 pole at which the sun had set, would still be within the region — or 

 zone as it might be termed — of twilight; and all places within a de^ 

 terminate distance of that pole must revolve within this same region 

 and thus, during their whole diurnal rotation, the inhabitants, if any 

 of such places, would experience a continued twilight of variable in 

 tensity. This circum polar region must be most extensive within the 

 arctic circle, about the 17th of October, and the 24th of February 

 and within the antarctic circle, about the 14th of April, and the 29th 

 of August: allowance being made for the encroachment upon the ter- 

 minator due to refraction, the sun's semidiameter, &c. 



III. On Temporary Stars. 



Prof. A. gave a brief statement of some of the hypotheses which 

 had been devised to account for the phenomena presented by these 

 bodies, and then suggested the following modification of one of 

 them : — 



