pressure, and therefore performing no work. He
thus established anew the relation between the heat consumed and the
work done. From five different series of experiments he deduced five
different mechanical equivalents, the agreement between them being far
greater than that attained in his first experiments. The mean of them
was 802 foot-pounds. From experiments with water agitated by a
paddle-wheel, he deduced, in 1845, an equivalent of 890 foot-pounds.
In 1847 he again operated upon water and sperm-oil, agitated them by a
paddle-wheel, determined their elevation of temperature, and the
mechanical power which produced it. From the one he derived an
equivalent of 781.6 foot-pounds; from the other an equivalent of 782.1
foot-pounds. The mean of these two very close determinations is 781.8
foot-pounds.

By this time the labours of the previous ten years had made Mr. Joule
completely master of the conditions essential to accuracy and success.
Bringing his ripened experience to bear upon the subject, he executed
in 1849 a series of 40 experiments on the friction of water, 50
experiments on the friction of mercury, and 20 experiments on the
friction of plates of cast-iron. He deduced from these experiments
our present mechanical equivalent of heat, justly recognised all over
the world as 'Joule's equivalent.'

There are labours so great and so pregnant in consequences, that they
are most highly praised when they are most simply stated. Such are
the labours of Mr. Joule. They constitute the experimental foundation
of a principle of incalculable moment, not only to the practice, but
still more to the philosophy of Science. Since the days of Newton,
nothing more important than the theory, of which Mr. Joule is the
experimental demonstrator, has been enunciated.

I have omitted all reference to the numerous minor papers with which
Mr. Joule has enriched scientific literature. Nor have I alluded to
the important investigations which he has conducted jointly with Sir
William Thomson. But sufficient, I think, has been here said to show
that, in conferring upon Mr. Joule the highest honour of the Royal
Society, the Council paid to genius not only a well-won tribute, but
one which had been fairly earned twenty years previously. [Footnote:
Lord Beaconsfield has recently honoured himself and England by
bestowing an annual pension of 200 pounds on Dr. Joule.]

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XX. THE COPLEY MEDALIST OF 1871.

DR