156 G. H. KNIBBS. 



for the computation of the coordinates of the direction of the sun's 

 path. He computed the intersections of great circles determined 

 by the proper motions of Aldebaran, Capella, Sirius, Procyon, 

 Pollux, Arcturus, a Lyrse, and a Aquilse, as given by Zach 1 from 

 comparisons of Bradley's places with Maskelyne's, and of Mayer's 

 with Piazzi's. Biot's system of axes was identical with Airy's 

 hereinafter mentioned. His theoretical expression for the "secular 

 parallax," analogous to that for parallax in altitude, contained 

 the factor, — the solar motion divided by the distance of the star. 

 The absolute values of these he said, were impossible of determin- 

 ation. Selecting Sirius, Procyon, and Arcturus as stars whose 

 proper motions were most suitable for a determination, he found 

 the point towards which our system tends, to have the following 



coordinates, viz. 



R.A. = 245-°0, D.= +36-°4 



Biot shewed that the evidence indicated displacements among the 



stars themselves : i.e., the changes of apparent place could not be 



wholly due to the sun's motion. 



(19) Herschel, 1805.— In May of the same year (1805), William 

 Herschel 2 returned to the problem, Dr. Maskelyne's proper 

 motions of 36 stars, the table of which he published in 1790, 

 affording the necessary material for a more elaborate computation 

 than he had at first undertaken. Herschel stated that the possi- 

 bility of solar motion had been shewn upon theoretical principles 

 by Dr. Wilson of Glasgow, and the probability, by Lalande, to 

 which latter reference has already been made. He assigned for 

 the point of direction, the position 



R.A. = 245°9, D.= + 49 k °6 

 this being based upon the proper motions of 6 stars only, and so 

 determined that the sum of the true proper motions should be a 

 minimum. He discussed also the quantity of the motion. 



(20) Herschel, 1806. — On February of the year following, (viz., 

 1806) Herschel read a further paper "on the quantity and velocity 



1 Tabular speciales aberrationis et nutationis, etc. 



2 Phil. Trans. Reprint Vol. xxin., pp. 233 - 256, 1805. Also Berl. Astr. 

 Jahrb., 1811, p. 224. 



