6 GOULD — REDUCTION OF D AGELET S OBSERVATIONS. 



3' 30". 94. The readings wero made for each system by means of verniers, which indicated 

 one-twentieth of each interval, or 15", for the first-named ; and one-sixteenth of each interval, 

 or a little more than 13". 18, for the second. Farther precision was attained by estimation in 

 the first system, and by a micrometer indicating seconds of arc in the second. Unlike the 

 zone observations of Lalande, those of d'Agelet give in most cases the readings according 

 to both systems, the two thus serving to check each other for the detection of large errors, 

 and their mean giving more accurate results than could be afforded by either alone. The 

 reticule was provided with three vertical metallic wires, at intervals of about 23s. of equatorial 

 time. 



The defects of the limb of the quadrant, by which it deviated from a plane, have often 

 been referred to, and Lalande has in various places 1 given different tables for the correction 

 to bo applied to times of transit at different altitudes. These tables, however, are not 

 accordant with one another, are quite rough, and by no means correct. Indeed, Lalande 

 himself 2 says of them, that they are only approximate, and that to determine properly the 

 position of an unknown star, it is necessary to calculate the right-ascensions of one or two 

 known stars, observed nearly in the same parallel. 



I have endeavored to elicit the nature and magnitude of the deviation from a vertical 

 plane , and thus to form a general table applicable to all of d' Agelet' s observations. It is greatly 

 to be regretted that we have no notes or memoranda on the subject from d'Agelet himself, 

 inasmuch as the care and assiduity, with which all his observations were evidently made, forbid 

 any doubt that this important point was made a subject of investigation. The nature of the 

 distortion now discovered being such as to excite suspicions that constant errors due to the 

 same irregularities might exist in the readings of zenith distance, the accuracy of the limb in 

 this respect also has been tested, and the suspicions are found to have been correct. The 

 detailed results of these investigations are given in the present memoir. 



$ 4. OBSERVATIONS. 



The published observations in the Histoire Celeste are given in ten columns. The first 

 shows the name of the star when known ; the second, the magnitude for perhaps one-half the 

 stars observed ; and the third, the approximate right-ascension to minutes of time, for a 

 sufficient number to allow of ready identification. Columns 4, 5, and 6 give the times of 

 transit across three wires, no attempt being made to observe more closely than to quarters of 

 a second — the decimals, .2, .5, and .7 being the only ones which occur. During the period of 

 must of the observations the clock indicated approximate mean time, and even after it was 

 changed to a sidereal rate it was not made to indicate sidereal time, but whenever it was 

 stopped for any reason it was always put in motion again without touching the hands. 3 

 Columns 7 and 8 contain the two readings of the limb. Column 9, entitled "Eeduction," 



1 Hist. C(U., pp. xi, 480; Mom. de l'Acad., 1785, p. 268—1789, p. 042. 

 8 Hist. (VI., p. 48U. 

 Mi im. de l'Acad. 1789, p. <14:s. 

 (6) 



