12 GOULD REDUCTION OP d'AGELET's OBSERVATIONS. 



§7. BASIS OF REDUCTION. 



A very cursory examination of the original observations will suffice to show the hopeless- 

 ness of any attempt at reduction by other methods than purely differential ones. Not only is 

 the quadrant an instrument ill adapted, at the best, for the determination of right-ascensions, 

 but d'Agelet' s quadrant proves to have been peculiarly irregular in form. The simplest 

 essays at determining the azimuth of its plane show the futility of any such attempt by ordinary 

 methods ; and the observations* being isolated and independent, preclude the facilities which 

 zone-reductions afford. No record is to be found of any endeavors by d'Agelet himself to 

 determine the position of his quadrant with reference either to the meridian or the nadir-point. 

 A few scattered notes, mentioning the apparent index-error, or the approximate correction to 

 the time of transit, as given by some particular star, constitute the sum total of the explicit 

 information recorded on these subjects. 



A means of obtaining the requisite data for a differential reduction was at hand in the 

 "Time-Star List," prepared by me for the use of the U. S. Coast Survey, and of which a 

 revised and improved edition 1 was published in 1862. This list contains 132 stars, well adapted 

 for use in determining time by observers in the northern hemisphere ; and gives the right- 

 ascensions and proper motions deduced by the method of least squares from the best recorded 



o 



observations, after referring them to the equinoctial points of Argelander's Abo Catalogue 2 , 

 slightly modified to conform to Peters's value of the nutation. Since the publication of this 

 list of right ascensions, I have computed the declinations in a similar manner, and thus the 

 data for an accurate determination of the positions at the time of d'.Agelet's observations were 

 before me. 



Of these stars, not only were 112 found among those observed by d'Agelet, but, most 

 happily, a considerable number prove to have been observed by him on almost every night ; 

 there being but one date when any fixed stars were observed without some one of these occur- 

 ring among them, and an ample number being found on all those dates when a large number 

 of stars were observed. On about one-half of the nights the number of standard stars was 

 not less than six, and on two-thirds of them it was not less than four. 3 



The dates of observation and the standard stars available for the reduction are exhibited 

 in the following table, in which the stars are designated by their respective numbers in the 

 Time-Star List. 



'Standard Mean Right-Ascensions of Circumpolar and Time Stars.— Washington, 1862. 

 2 DLX Stellarum Fixarum Positioned Media?, ineunte anno 1830.— Helsingfors, If-::.".. 



' '" bservations of (he 22d June, 1784, are not counted, having continued lor less than ten minutes, and comprising but 



itars. ° 



four stars 



(12) 



