Sun’s observed and computed right ascensions , &c. 437 
ascensions, varies only one thousandth of a second, from the 
quantity obtained on the 4th, when the instrument was en- 
tirely defended from the solar rays. 
On September 24th, the instrument was completely screened 
from the sun's rays ; but on the 25th they were allowed to 
fall upon the instrument's western axis and brace, sixty-three 
minutes, during a cloudless sky;* yet between the results of 
the one day, and the other, there is only a difference of 
7 hundredths of a second. 
On October 21st and 22nd, the instrument being exposed 
to the sun’s rays, thermometers under the black covers of the 
western axis and brace, differed on the former day 12°.5 from 
those under the covers of the white axis and brace ; but on 
the latter, the difference of temperature was more than 1 6° ; 
the difference between the results of the two days' observa- 
tions, is nine hundredths of a second : unfortunately, there are 
no observations with which these can be compared. 
In like manner might we discuss individually, the results of 
experiments made on several occasions, during the year 1822 ; 
the days however are noted in Table VI. when the instru- 
ment was exposed ; and Table VII. details all the particulars 
which are essential to the investigation ; to which therefore 
the reader is referred, as also for a more circumstantial 
account of the exposure of the instrument to the sun’s rays, 
during the year 1821. 
On looking down the columns of differences, between the 
observed and computed right ascensions of the sun, from the 
* Experiments during exposure of the instrument, were never commenced, ex- 
cept under every probability of success; when however (as frequently happened), 
transient clouds obscured the sun, even but for half a minute, the operations were 
discontinued, and the results disregarded and destroyed. 
mdcccxxvi. 3 L 
