155 
transit instruments by the effect of temperature. 
taking place according to the explanation I gave of it in the 
Transactions for 1825 and 1826. According also to the 
principle of that explanation, and the preceding experiments, 
it follows, that the partial heating of the braces on the same 
side, or the partial heating of the upper or lower braces, 
produce counteracting effects, and may, in certain instru¬ 
ments, under certain degrees of temperature, produce balanc¬ 
ing effects, as far as the time of the star’s passage is concerned. 
This may be the case with Mr. South’s instrument, and if 
so, we should at once have an explanation of its seeming 
inflexibility when exposed to the sun. I hope I am altoge¬ 
ther within the bounds of courtesy and fair criticism, when I 
remark, that I see nothing in Mr. South’s experiments that 
force me to the conclusion of his instrument being exempt 
from those infirmities with which mine is afflicted. 
In these experiments of Mr. South’s on Polaris, the braces 
on the same side, whether the eastern or western, are of the 
same temperature. The retardation, or acceleration, there¬ 
fore of the star’s passage, arises from the difference of the 
changes produced by temperature in the upper and lower 
parts of the tube of the telescope ; which difference, estimat¬ 
ing it by its effect on the time of the star’s passage, may be 
very small, or not greater than the error of observation, or, 
(for there is nothing improbable in the supposition) insensible. 
But it may be said, is not this the bending of facts to suit an 
hypothesis, instead of adopting the more natural supposition, 
that the steadiness of the instrument arises from the excel¬ 
lence of its construction. The construction, indeed, of Mr. 
South’s instrument, in what regards the putting together of 
the tubes composing the telescope and axis, essentially differs 
