This two-day chronometrical thermometer has reversed bimetal rims on its balance. This design meant that its timekeeping would be subject to changes in temperature. George Airy, 7th astronomer Royal, ordered this instrument from Molyneux, to provide a more meaningful reference on the effects of temperature changes on chronometers than that would be achieved by taking readings from a thermometer.


On 19 January 1841 the Astronomer Royal George Airy wrote to the Hydrographer of the Navy, Captain F Beaufort RN: I have long been thinking of using, for fine and accurate registry of the compensation of our chronometers (a thing daily becoming more important), either a good chronometer with plain balance, or a good chronometer with a balance in which the metals of the compensation arms are reversed, so as to increase the reactance[?difficult to read] to change of temperature. Molyneux would make the former for 16 guineas, the latter for 19 guineas. 1st. What do you think of the scheme? 2nd. If you want I should approve of it, is it in your superintendence or in mine, to recommend this expense to the Board? Beaufort replied the following day: As your proposed Chronometric Thermometer is mainly for the use of the Chronrs I can have no hesitation in undertaking the consent of the Board. Be so good therefore as to order it from Molyneux - I presume that the inversion of the metals in the balance will give the instrument very much greater susceptibility, and that you will therefore decide upon that.

The Chronometrical Thermometer was thus ordered from Molyneux, and was put into immediate use in the annual trials of 1841/42. Airy recorded in his Astronomer Royal’s Report for 1842, concerning rates of chronometers: Great certainty will be given to all determinations of this nature at the Royal Observatory by the use of a Chronometrical Thermometer … suggested and constructed by Mr R. Molyneux, which gives with the greatest delicacy the summation of the daily or weekly effects of temperature. This instrument went on to provide ‘thermo-chronometric’ data every year for the annual chronometer trials at Greenwich for the whole of the rest of the trials, up to 1915. The denomination ‘No.3’ suggests that Molyneux made at least three of these instruments, though the letters quoted above suggests only one was purchased at that time. This is the only instrument of the kind which has come down to us from the Observatory’s equipment, and it is assumed to be the one used in all the above trials.

The instrument has been in the collection since at least the early 1970s. It was loaned to the Physics Exhibition at Alexandra Palace in March 1972 [56th Exhibition of Scientific Instruments and Apparatus (Institute of Physics) at Alexandra Palace in London from March 13-16, 1972].