364 Dr. Herschel’s Account of the Changes that have happened 
“ joining Castor and Pollux in the heavens, seen by the naked 
“ eye.” 
Dr. Maskelyne informs me, that the observation must have 
been made about the year 1759; and also mentions, that he 
himself verified the fact, as to the line joining the two stars ap- 
pearing through the telescope parallel to the line joining Castor 
and Pollux, in 1760 or 1761 ; but that he did not examine it at 
various times of the year. 
The advantage of having an angle of position observed in 
1759 by Dr. Bradley, and so soon after verified by Dr. Mas- 
kelyne, will give us an addition of 20 years to our period. On 
calculating the right ascension and polar distance of Castor and 
Pollux for November 5, 1759, it appears, that a line drawn 
from Pollux' through Castor, must have made an angle of 
56° 32' north preceding with the parallel of that star ; and, this 
being also the position of our double star, we have an interval 
of 43 years and 142 days, for a change of 45 0 39', from the time 
of Dr. Bradley’s observation to that of my last measure of the 
angle. By this we are now enabled to correct our former cal- 
cul tion, which was founded upon a supposition that the first 
angle of position I had taken was perfect ; but this could hardly 
be expected, and on examination it appears that the measure 
was 2 0 40' too little. The annual motion, by our increased period, 
is i° 3 y ,i ; and the computation of the angles of position in the 
3d column of the following Table, as well as the differences 
contained in the 4th, are made according to this motion. 
