parallax of certain fixed stars. 289 
the observations given would be increased o",s, and his 
summer distance decreased by about the same quantity ; 
which would make his results differ in the same direction as 
they should do by the effect of parallax. I do not intend by 
this that any argument in favour of parallax can be deduced 
from his results, but only to show the effect of small uncer- 
tainties. 
There may be uncertainty as to the stability of the instru- 
ment during the interval which elapses between the successive 
observations of « Cygni and (3 Aurigag, which is sometimes 
necessarily of several days. 
This is the point before alluded to ; and there appears, on 
examining Mr. Pond’s results as to <*. Cygni and Q Aurigag, 
indications of such an instability, and that to an amount that 
may do away the conclusion he has drawn from these ob- 
servations. 
The seconds of the micrometer for the same star should 
be the same in summer and winter, after the usual reductions, 
supposing no uncertainties in the elements of these reduc- 
tions, supposing no parallax, and supposing no derangement 
in the instrument. Now, referring to Mr. Pond’s paper, the 
seconds for a Cygni are decreased by about 5" in summer, and 
those of (3 Aurigag increased by nearly the same quantity. 
This may be concluded to arise from a derangement in the 
instrument by the change of temperature, as Mr. Pond has 
mentioned no other cause. The effect of an increase of 
temperature, therefore, appears to be to decrease the seconds 
of a Cygni, and to increase those of (3 Aurigag. Applying this 
to observations of the same day in winter, a Cygni passes the 
meridian near noon, and (3 Aurigag near midnight, or at least 
