DEPARTMENT OF MERIDIAN ASTROMETRY.* 



Benjamin Boss, Director. 



The general scheme of operations of this Department has been 

 continued during the year September 1915 to September 1916, the 

 period covered by this report. In general this scheme consists of the 

 preparation of material for the detennination of accurate systematic 

 proper-motions and the discussion of the proper-motions already pre- 



INVESTIGATIONS. 

 PREFERENTIAL MOTION ACCORDING TO TYPE. 



Mr. Raymond has nearly completed the investigation of preferential 

 motion mentioned in the last report (Year Book, 1915, p. 242) ; and the 

 results, together with a description of the method, will soon be pub- 

 lished in the Astronomical Journal. 



The following tables show the more important of the results. Table 

 1 gives, following the designations of the various groups of stars and 

 their numbers, the elements A, D, and M of solar motion, and the 

 right ascension and declination of the three principal axes of stellar 

 motion. Table 2 gives the mean-square velocity Xi, X2, X3, parallel to 

 each of the three principal axes. If the velocity components in each 

 direction are distributed according to the error-law, the velocity-figure 

 is a generalized Schwarzschild's ellipsoid; and the mean velocity com- 

 ponents in the three directions are given hy u = V 2Xi/7r, v = ^2X2/^, 

 w = ■\/2X3/7r. The proper-motions, and hence u, v, w, and M, are in 

 seconds of arc per century, and so are affected by the mean parallaxes 

 of the various groups. The quantities u/M, v/M, w/M are practically 

 free from this effect, and are given in the last three columns of table 2. 



The stars used include all in the Prehminary General Catalogue 

 with proper-motions under 20" per century, except some discarded for 

 cause, such as too small weight or companions of brighter stars. 



The distribution of the 493 stars of type B is so irregular that it can be 

 handled by this method only after special treatment. This remains 

 to be done, but the result will probably not be of great value when 

 found. The material is included in the general group ''AH." '*X" 

 includes stars too faint to appear in the Draper Catalogue. These 

 faint stars are generally supposed to belong in large part to "late" 

 types, and the systematic motions of this group resemble those of 

 "late" rather than "early" types. Some conclusions to be drawn 

 from these tables follow: 



(1) The motions of every group show unequal preference for three 

 cardinal directions. The three principal axes of inertia of the velocity- 

 figure are unequal. 



♦Address: Dudley Observatory, Albany, New York. 



217 



