parallax of certain fixed Stars. 285 
7. As to v, or the variation in declination. This consists of 
two parts, one the effect of the precession of the equinoxes, 
and the other of the proper motion of the star. The former 
seems determined with sufficient accuracy. Also as far as 
regards the stars of the standard catalogue of Mr. Pond, the 
latter seems pretty well agreed on among astronomers. But 
here arises a question of some importance : is the proper mo- 
tion of each star uniform ? It is assumed to be so in computing 
it by two results separated by a long interval. A series of 
results sufficiently accurate, and separated by intervals suf- 
ficiently long, have not yet been obtained to ascertain this 
important point. 
A star of the 6th magnitude, 1 Leporis, seems to furnish an 
instance of a variable proper motion, by a. comparison of the 
observations of Bradley and M. Piazzi. There is nothing 
against a variable proper motion in our theories of the nature 
and motions of the fixed stars. Hence, another source of un- 
certainty in computing the index error. 
8. Lastly, as to c, or the mean polar distance in the standard 
catalogue. This is subject to two uncertainties. The ori- 
ginal error in the catalogue, and an uncertainty in the annual 
variation, as mentioned in the last article. 
Notwithstanding all the care that has been used by Mr. 
Pond in perfecting his standard catalogue, it may contain 
small inaccuracies, as will easily be apprehended from the 
observations in the preceding articles. 
The uncertainties to which the index error is liable from 
the above causes, are independent of those to which the 
observation to which it is applied, is also subject. It may be 
