Paper, 'On a compensated air thermometer' by Hugh Longbourne Callendar Callendar writes: 'In a paper which I had the honour to present to the Royal Society some four years ago “On the Practical Measurement of Temperature,” I described in detail a somewhat elaborate form of air thermometer with which it was found possible to attain an accuracy of the order of 0.01° C. I have since succeeded in overcoming some of the difficulties encountered in that investigation, and in evolving on similar lines a form of instrument which is capable of a much higher order of accuracy, and which has the further advantage that both the observations and the calculations are immensely simplified.' Annotations in pencil and ink. Subject: Scientific apparatus and instruments Received 29 October 1891. Read 10 December 1891. Communicated by Joseph John Thomson. A version of this paper was published in volume 50 of the Proceedings of the Royal Society as 'On a compensated air thermometer'. 1891