Sun’s observed and computed right ascensions, &c. 439 
the mean of 5 observations, made when the instrument was 
entirely defended from their influence, six thousandths of a 
second of time; whilst in 1822, the mean derived from 8 ob- 
servations made underexposure, compared with the mean of 
8 results, obtained when the instrument was completely de- 
fended from the sun's rays, differs sixty-nine thousandths of 
a second of time. 
The mean therefore of the two series, allowing each, a 
weight proportional to the number of observations on which it 
rests, is forty-five thousandths of a second of time. Whether 
this arise, from error of observation, erroneous computation, 
or from instrumental derangement, we have not sufficient 
data* to determine: fortunately, however, the quantity is 
very small, and if it really could be brought, to support the 
hypothesis, “ that the sun's rays falling unequally upon the 
instrument, occasioned the discordances complained of, " it 
would lose much of its apparent weight, when it is remem- 
bered, that not the ordinary exposure of the instrument to 
the sun, but ten times that quantity, was employed to pro- 
cure it. 
The mean difference however between the observed and 
computed right ascensions is less under exposure, than when 
the instrument was defended ; hence, were it wanted, it might 
be called upon as additional evidence, in favour of the con- 
clusion which the experiments afford, namely, “ that the 
discordances between the observed and computed right ascen- 
* On referring to page 446, there seems some reason to believe, that the differ- 
ences found between the observations of February 28th and March 1st, May 31st 
and June 1st, June 3rd and June 4th, June 6th and June 7th, are not the results of 
instrumental derangement, nor of erroneous observation. The Greenwich and 
Paris observations corroborate our 1st difference ; the mean of the Greenwich and 
Paris, supports our 2d; the Paris determination coincides with our 3d; and the 
Dublin is nearly similar to our 4th. 
