136 
SECOND REPORT - 183 #. 
paring these places with those obtained from Bradley's obser¬ 
vations, as reduced by Dr. Maskelyne. 
The determination of the mean declination of a star is inde¬ 
pendent of other observations, and depends only on the refrac¬ 
tion and nutation at the time of observation (as in a series of 
observations the effect of aberration is nearly eliminated). For 
the mean 7R, the observations are not independent; the posi¬ 
tion of the different stars with respect to each other is the 
subject of one determination, and that of all with respect to the 
sun, when he has a particular zenith distance, is the subject of 
another : the values of iR are affected, therefore, by errors of 
refraction, as well as by nutation, and a complication of errors 
of observation. Maskelyne had used Bradley’s table of re¬ 
fractions, and had used 9 "'55 as the coefficient of nutation in 
declination. The right ascensions were found by comparing 
all the other stars with a Aquilse, and comparing a Aquilas with 
the sun. 
At the beginning of the century Cagnoli determined inde¬ 
pendently the place of Capella, and founded on this determina¬ 
tion a catalogue of stars of which I shall speak again. 
In 1806 Mr. Pond gave a catalogue of N.P.D. founded on 
his observations with an altitude-and-azimuth instrument, but 
using the same corrections as Maskelyne. 
In 1807 Piazzi published, in the sixth volume of the Palermo 
Observations, his catalogue of 120 principal stars observed 
with great care, and a greater number of stars on which less 
attention was bestowed. All the places were referred to Pro- 
cyon and a Aquilse ; and these stars were compared immedi¬ 
ately with the sun. The table of refractions used by Piazzi 
was one deduced by himself from observations of circumpolar 
stars in different parts of their diurnal circles, and differed little 
from Bradley’s. This catalogue therefore was strictly inde¬ 
pendent. In 1814 it was extended so as to include 7646 stars, 
and published separately; and this large catalogue is at this 
time referred to by all observers as a standard catalogue. 
The subject of refraction had in the last century attracted 
considerable attention, and had been treated theoretically by 
Oriani, Kramp, and Laplace. The object of these writers was, 
from an assumed law' of constitution of the atmosphere, founded 
as far as possible on experiment, to determine, a priori, the 
magnitude and law of refraction. The experiments made by 
the French chemists and opticians had determined the relation 
between the pressure and density of the air (subject to a very 
small doubt as to the effect of heat,) and the quantity of refrac¬ 
tion, without reference to any astronomical observations. Some 
