parallax of certain fixed stars . 297 
polar distances of stars having nearly the same right ascen- 
sion, and north polar distance. 
It would be an interesting circumstance, could the existence 
of visible parallax in any one star be ascertained, and placed 
beyond doubt, by the joint results of two separate instru- 
ments. The comparison of my summer and winter observa- 
tions of a Aquilae indicating so great a parallax, induces me 
to expect that as to this star it may yet be accomplished. 
Mr. Pond suggests that the effects of refraction may occa- 
sion some uncertainty as to this star. This can only arise 
from irregularity of refraction ; and it seems scarcely possible 
that the mean of 100 observations can be sensibly affected 
thereby. My refractions have been computed from the 
internal thermometer placed on the instrument : had they 
been computed by the external thermometer, the difference 
between the summer and winter zenith distances of a, Aquilas 
would have been lessened about €>",3. As a, Aquilas passes 
the meridian near noon in winter, there is seldom much 
difference then between the external and internal thermo- 
meter here. 
If the discordance which I have found between the summer 
and winter zenith distances had arisen from the different 
temperatures at the two seasons, it might have been expected 
that Aldebaran, Capella, a. Orionis and Procyon would have 
been much more affected by this cause ; as in winter they 
pass the meridian at night, and in summer in the day time; 
and therefore as to these the observations are made in the 
extremes of temperature. 
To many, the time and labour spent in this minute enquiry, 
may appear wasted. Some however will justly appreciate 
