202 CELESTIAL MEASURINGS AND WEIGHINGS. 



to the celestial vault and carried round with it, such a 

 thing is not to be supposed. In the total absence then 

 of any information as to the velocity or direction of the 

 real motions, we can only presume that such as appear 

 to move fastest are nearest. The fact may be otherwise, 

 but such at least is the prima facie presumption. Now, 

 while the stars in general exhibit an annual apparent 

 proper motion averaging less than a second per annum, 

 these two 61 Cygni and a Centauri, are carried annually 

 from their places, by movements apparently rectilineal 

 of 5"'3 and 3'"6 respectively : motions which would carry 

 them away from their places through a space equal to 

 the moon's apparent diameter in 339 and 499 years re- 

 spectively. In point of fact, we find that they are nearer, 

 so that a part at least of their great apparent motions is 

 owing to proximity. 



(27.) Such a uniformly progressive change of place 

 complicates apparently, but not really, the microscopic 

 process we have described. Being accurately known by 

 long continued observation, both in amount and direc- 

 tion ; its effect in displacing the star among its neigh- 

 bours is easily taken account of and allowed for. The 

 combination of these two motions, the one real and 

 rectilinear, and the other apparent and elliptic, will be 

 readily understood from the accompanying diagram, 

 where ab, be, cd represent the former continued for 

 three years ; e, /, g, the ellipses described in those years 

 in virtue of the latter in the direction of the arrows ; and 

 h i k the sort of undulating line apparently described in 

 virtue of them both going on together. 



