On the derangements of certain transit instruments by the effects of temperature

In the Philosophical Transactions for 1825, the author alluded to the derangement of the Cambridge transit instrument, arising from unequal expansion of its braces, establishing, as he conceived, the fact and cause of such derangement; and in a subsequent paper instanced its effect in one case as altering, by no less than 20", the time of the passage of the pole star over the wires. In consequence the removal of the braces was resolved on, but from one cause or other delayed; but the author considers good to have arisen from this procrastination, as enabling him to make further experiments, which he was led to do in consequence of Mr. South’s observations, which lead to conclusions opposite to those deduced by himself. To satisfy his own mind, therefore, he instituted the series of experiments described in this paper. His first care was to determine precisely, by a series of transits, the polar intervals between the wires of his eye-piece. He then observed the pole star at its lower culmination, and after its passage over the middle wire, applied a warm blanket to the upper eastern and lower western brace, and found that a deviation of the telescope to the west had taken place, such as to alter the passage over the remaining wires nearly 19 seconds, and in the direction corresponding to the expansion of the braces. Another observation, under more favourable circumstances, gave a similar result; viz. 18 seconds of retardation.