481 
the parallax of a. Lyra. 
“ Although this period embraces only half the interval in 
“ which the greatest change, or double parallax, is effected, 
“ a circumstance which at first may appear very disadvan- 
“ tageous, yet that is more than compensated, in my opinion, 
“ by the number of observations, and by a uniformity of 
“ temperature, such as never can be expected in the extreme 
“ seasons of winter and summer/' 
On the contrary, it appears to me, that inconvenient cir- 
cumstances occur in this interval, comprising the latter part 
of summer and the commencement of autumn. The star 
then passes the meridian after sun-set, at which time, often 
the greatest difference exists between the external and in- 
ternal temperatures. 
At that time of the year, on clear nights, after sun-set, 
great degrees of cold often suddenly take place in the open 
air, and it is almost impossible to equalize the temperature. 
In winter, when a Lyrae passes in the middle of the day, 
there is seldom, as has been before said, much difference of 
external and internal temperature, except in extreme cold. 
To which may be added another point of importance : it 
is much more difficult to bisect a. Lyrse when it passes after 
sun-set, than when it passes in day-light 
But the real strength of the argument, from these new 
observations of u Lyrae, lies in comparing those made after 
the paper was read, with those made in July and August, 
Here the Dublin and Greenwich instruments are completely 
at variance. 
The Dublin instrument has shown, by a great number of 
observations, continued for several years, the double zenith 
mdcccxxiv. 3 Q 
