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 of water. These were communicated to the British Association at Oxford in June 1847. They led to the result 781.5. After the reading of this paper Joule and Sir William Thomson first met, and the acquaintance, to use Sir William's words, ‘quickly ripened into a life-long friendship.’

Joule's own account of this meeting, and of the general reception of his work at this time, is given in a note, dated 1885, to his ‘Collected Papers’ (ii. 215): ‘It was in 1843 that I read a paper “On the Calorific Effects of Magnetic Electricity and the Mechanical Value of Heat” to the chemical section of the British Association assembled at Cork. With the exception of some eminent men, among whom I recollect with pride Dr. Apjohn, the president of the section, the Earl of Rosse, Mr. Eaton Hodgkinson, and others, the subject did not excite much general attention, so that when I brought it forward again at the meeting in 1847, the chairman suggested that as the business of the section pressed I should not read any paper, but confine myself to a short verbal description of my experiments. This I endeavoured to do, and discussion not being invited, the communication would have passed without comment if a young man had not risen in the section, and by his intelligent observations created a lively interest in the new theory. The young man was William Thomson.’

Sir William Thomson says in a letter to Mr. Bottomley (Nature, 1882, xxvi. 619) that at first he thought Joule must be wrong, but as he listened he recognised that ‘Joule had certainly a great truth, and a great discovery, and a most important measurement to bring forward.’ He continues: ‘Joule's paper at the Oxford meeting made a great sensation. Faraday was there, and was much struck with it, but did not enter fully into the new views. It was many years after that before any of the scientific chiefs began to give their adhesion. It was not long after when Stokes told me he was inclined to be a Joulite.’

About a fortnight later Joule and Thomson met again by chance near Chamounix. Joule had just married, and was on his wedding tour, carrying a long thermometer, with which he was going to try for a rise of temperature in waterfalls, and the two arranged to make an experiment a few days later at the Cascade de Sallanches, but found it too much broken with spray. On his return to Manchester, encouraged, no doubt, by the reception of his work at Oxford, and aided by the generous enthusiasm of Thomson, Joule set himself to repeat his experiments on the production of heat by friction. The results were communicated to the Royal Society by Faraday on 21 June 1849, and printed during the following year in the paper ‘On the Mechanical Equivalent of Heat’ (Phil. Trans. 1850, pt. i.; Collected Papers, i. 298). The introduction to the paper contains a very fair account of the labours of others in the same field. A long series of observations, conducted with the utmost care, leads to the result that ‘the quantity of heat capable of increasing the temperature of a pound of water (weighed in vacuo, and taken at between 55° and 60° Fahr.) by 1° Fahr. requires for its evolution the expenditure of a mechanical force represented by the fall of 772 lb. through the space of one foot,’ or, in more modern phraseology, we should say, the expenditure of 772 foot-pounds of mechanical energy.

For nearly thirty years this result of Joule's stood alone as the one satisfactory determination of a most important physical constant. Writing in the ‘Proceedings’ of the American Academy for Arts and Sciences, 11 June 1879, Professor Rowland of Baltimore says: ‘We find that the only experimenter who has made the determination with anything like the accuracy demanded by modern science, and by a method capable of giving good results, is Joule, whose determination of thirty years ago, confirmed by some recent results to-day, stands almost, if not quite, alone among accurate results on the subject.’ Professor Rowland proceeds to explain the reasons why he undertook fresh experiments, and concludes that the difference between his own results and those of Joule is ‘not greater than 1 in 400, and is probably less.’

Researches on various subjects more or less cognate to the above continued to occupy Joule for some time longer. In 1840 Joule had himself established the connection between the work required to produce an electric current in a wire and the heat evolved. Sir William Thomson's papers on the dynamical theory of heat and various allied subjects were published in 1851 (Trans. R. S. E., 1851), and in a paper ‘On Applications of the Principle of Mechanical Effect to the Measurement of Electro-motive Forces and of Galvanic Resistances in Absolute Units’ (Phil. Mag. December 1851), he pointed out that Joule's measurements of 1840, combined with a knowledge of J., gave a means of measuring in absolute units the electrical resistance of the wire employed by him, or that conversely if the resistance of the wire were known absolutely the measurements could be used to determine J. The question of absolute electrical units was brought into