157 
transit instruments by the effects of temperature. 
it likely it should: it might render, under the same circum¬ 
stances, the derangement less, or it might render it greater; 
for this is a point which I think should be determined solely 
by experiment. The experiment which I have just quoted, 
apparently renders the derangement greater; but this is a 
kind of result on which I lay no stress, since I have no 
grounds for not knowing that the heat applied at Greenwich 
was not three or four times what I applied. 
From my own experiments, and from the testimony which 
I have just adduced from Greenwich, I find it, then, extremely 
difficult to believe that Mr. South’s transit instrument should 
not obey the influence of expanding braces. I have stated 
some grounds for suspecting that it is not endowed with that 
inflexibility which it appears to assume in some of the obser¬ 
vations. Not one of these, in my opinion, bears directly upon 
the point in question. A single observation of Polaris in 
October, with one brace alone of the instrument exposed to 
the sun, would have been a better test of its steadiness than 
all that are recorded. I invite that indefatigable and ardent 
observer to the trial of this test. 
I am unwilling, and indeed not quite prepared, to enter on 
the difficult subject of the sun’s meridional observations. The 
errors of the clock, or the difference of the sun’s observed 
transits, and of his computed right ascensions, do not agree, 
as it is known, with the errors found by the transits of the 
stars. The former errors are always less. In this general 
result, the observations I have made agree with the Greenwich 
and Dublin observations, and with Mr. South’s. The mean 
of the differences of the errors of the clock, as estimated by 
the sun and stars, is about six tenths of a second. And this 
