A. MATHEMATICS AND ASTRONOMY. 37 



found by Struve for the time of revolution and eccentricity, 

 and the other elements of its orbit. Of the thirty -eight 

 positions given from 1827 to 1874, only two cases occur in 

 which the discordances amount to one tenth of a second of 

 arc ; and these causes, it is promised, will be, at least in part, 

 explained away in a forthcoming memoir relating to the 

 peculiar systematic errors that attach themselves to the 

 observations made by Otto Struve in 1840-41. Of the re- 

 maining thirty-six discordances, eight slightly exceeded one 

 twentieth of a second of arc. The remainder are less than 

 that quantity. The probable error of a single observed dis- 

 tance or the result of a single night's work is 0.040. Obser- 

 vations of these stars made by other astronomers agree satis- 

 factorily with the orbit determined by Otto Struve, although 

 the average of the discordances is somewhat larger in their 

 observations than in his own. Notices of the Royal Astro- 

 nomical Society, Ufay, 1875, 372. 



METHOD OF CONSTRUCTING CHARTS OF STARS. 



In constructing the new charts of the stars in the neigh- 

 borhood of the ecliptic, the French astronomers, under the 

 general direction of LeVerrier, have adopted some novel and 

 excellent methods. The brothers Paul and Prosper Henry, 

 in that portion of the work which they have performed, 

 have made use of two equatorials, having apertures of about 

 nine inches, and by a duplicate examination of each portion 

 of the heavens have been able to discover many small plan- 

 ets and comets. The great equatorial of the observatory 

 has been furnished with a micrometer of special construc- 

 tion, in which advantage is taken of the precision with 

 which the telescope is made, by means of the regulator of 

 Foucault, to follow the diurnal movements of the stars. 

 This micrometer gives immediately the co-ordinates of any 

 star comprised in the field of view of the telescope, with 

 reference to a given standard point, and that in such shape 

 that these figures may be entered directly upon the chart. 

 This micrometer is also now being applied to the mapping 

 of the individual stars in some of the clusters. The accu- 

 racy of the work done with this instrument is such that the 

 star places given upon the charts are reliable within a sec- 

 ond of time and one tenth of a minute of arc: a result some- 



