Abstract
In the Philosophical Transactions for 1825, Part II. p. 418, I gave an account of the transit instrument belonging to the Observatory at Cambridge. Amongst other circumstances, I mentioned one of a derangement of the telescope arising from an unequal expansion of its braces. I established, as I thought, by direct experiments, the existence of such a derangement, and pointed out its cause. In a subsequent volume of the Transactions (1826, Part II. p. 75), I gave an instance of the quantity of such derangement caused by the sun's rays falling on the upper western brace. That circumstance caused a retardation of 20 seconds in the passage of Polaris at its lower culmination. The removal of the braces, which, after the above experiments I judged to be a necessary measure, has, from one cause or another, been delayed till this time. I have however derived some good from my procrastination, since it has enabled me to institute the experiments which I am now about to detail.
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