the effects of parallax, &c. of certain fixed stars. 345 
No. of Ob. 
N.P.D. Jan. i, 1819. 
Constant of 
aberration. 
a Cassiopeas 
ot. Cephei 
/ 3 Cephei 
87 
100 
62 
34 27 23,47 
28 IO 42,74 
20 13 57^5 
20,30— ,14 p 
20,56— ,39 p 
20,20 ,38 p 
In my paper, Philosophical Transactions 1819, the constant 
for « Ursce Majoris was given. In Table 1, (2 Ursae Majoris 
has been introduced ; both could not be observed at the same 
time ; and having formerly intended to deduce the constant 
of aberration from the mean of a great number of stars, 
(2 Ursae Majoris was observed. 
The importance of the enquiry relative to the velocity of 
light, has since induced me to multiply as much as possible 
observations of the same star, and therefore the observations 
of a Ursae Majoris have not been resumed. 
Lunar Nutation. 
A comparison of zenith distances of certain of the stars 
that I observed in the years 1809-1814, and of the zenith 
distances of the same stars observed in the years 1818-1820, 
has given the following results relative to lunar nutation. 
It is almost unnecessary to remark, that those stars only 
were used in which the nutation at each period was nearly a 
maximum, with contrary signs. Two circumstances are 
particularly required to obtain the most accurate results. 
1. That a comparison of results should be deduced from 
observations made by the same instrument. 
