368 Mr. Herschel’s and Mr. South’s observations of the apparent 
61 Cygni continued. 
mean motion deduced in like manner from a comparison of 
each of the remaining data with our mean result of 1822, 
comes out o°.yig6, a very satisfactory coincidence when the 
nature of such a mode of determination is considered. The 
mean of both gives a mean annual motion of o°.73o, in the 
direction spnf or direct. If we employ this to compute the 
position at the several times of observation, assuming that of 
1822 as correct, we shall have the following comparison : 
Date. 
Observed Position. 
Calculated Position. 
Error of Observation. 
1753.8 
54.6 nf 
0 
557 nf 
1.1 
I778.O 
89-o 
38.1 
— O.9 
1781.9 
36.2 
35-2 
1.0 
1 79S- 6 
37.2 
2 6.7 
— 10.5 
1800.0 
19-7 
22.0 
+ 2.3 
1805.0 
11.5 
184 
4* 6-9 
i 812,3 
10.9 
13.0 
■+ 2.1 
1819.9 
7.0 
7-5 
+ 0.5 
1822.9 
5-3 
5-3 
-f* 0.0 
The errors are not greater than might be expected when 
we consider that the most important of them, — io°.5, is 
that of a single observation of each star by Lalande, and 
that an error of 2" in the difference of declination would suf- 
fice to produce it. 
The mean angular motion of these stars then about their 
common centre of gravity is not far short of that of the two 
stars of Castor, while their apparent mutual distance is at 
