Sun’s observed and computed right ascensions , &c. 435 
north and south were about 18 or 20 inches in breadth ; and 
the telescope, when directed to the zenith, extended some 
way between the ceiling of the observatory and its roof. 
The shutters were so contrived as to be opened in an instant ; 
and by a slight frame-work it was very easy to screen all 
the parts of the instrument, and also the piers, from the 
access of the sun's rays ; it was likewise a matter of the 
greatest facility to prevent his rays from falling on the 
eastern half of the instrument, whilst the western was ex- 
posed to their influence. 
Previous to observing the sun's transit, it was my ordinary 
habit not to open the shutter, till his first limb had nearly 
reached the first wire of the instrument. This precaution 
was uniformly adapted, in the observations of 1821, till the 
2 2d of August ; the consequences are seen by the annexed 
differences. 
If, however, we adopt the hypothesis, that the mere ex- 
posure of the instrument to the sun's rays during the ob- 
servation of his transit (a period about 4^ minutes) be 
adequate to produce instrumental derangement corresponding 
to 8 or 9 tenths of a second in time, it is fair to expect that 
a longer exposure would produce a greater discordance, and 
vice versa. On the 2 2d of August, therefore, the western* half 
of the instrument was exposed to the solar rays, 18 minutes 
before the sun's centre came to the meridian ; the effect, 
however, being very inconsistent with theory, on the 23d it 
was exposed 24 minutes ; the mean differences of temperature 
of the western and eastern axes, and western and eastern 
* By the nature of the roof, and the construction of the interior of the observa- 
tory, independently of the shutter and screen, the sun’s rays could not fall on the 
eastern brace and axis, ’till the sun had nearly reached the meridian ; but the 
western brace and axis, towards the pivot, were accessible to his rays nearly i^hour 
before noon, provided the shutter was opened. 
