of a doctrine is often
practically sealed, and the truth of one is often practically
accepted, long prior to the demonstration of either the error or the
truth.

Perpetual motion was discarded before it was proved to be opposed to
natural law; and, as regards the connection and interaction of natural
forces, intimations of modern discoveries are strewn through the
writings of Leibnitz, Boyle, Hooke, Locke and others.

Confining ourselves to recent times, Dr. Ingleby has pointed out to me
some singularly sagacious remarks bearing upon this question, which
were published by: an anonymous writer in 1820. Roget's penetration
was conspicuous in 1829. Mohr had grasped in 1837 some deep-lying
truth. The writings of Faraday furnish frequent illustrations of his
profound belief in he unity of nature. 'I have long,' he writes in
1845, 'held an opinion almost amounting to conviction, in common, I
believe, with other lovers of natural knowledge, that the various
forms under which the forces of matter are made manifest have one
common origin, or, in other words, are so directly related and
mutually dependent, that they are convertible, as it were, one into
another, and possess equivalence of power in their action.' His own
researches on magneto-electricity, on electro-chemistry, and on the
'magnetisation of light led him directly to this belief. At an early
date Mr. Justice Grove made his mark upon this question. Colding,
though starting from a metaphysical basis, grasped eventually the
relation between heat and mechanical work, and sought to determine it
experimentally. And here let me say, that to him who has only the
truth at heart, and who in his dealings with scientific history keeps
his soul unwarped by envy, hatred, or malice, personal or national,
every fresh accession to historic knowledge must be welcome. For
every new-comer of proved merit, more especially if that merit should
have been previously overlooked, he makes ready room in his
recognition or his reverence. But no retrospect of scientific
literature has as yet brought to light a claim which can sensibly
affect the positions accorded to two great Path-hewers, as the Germans
call them, whose names in relation to this subject are linked in
indissoluble association. These names are Julius Robert Mayer and
James Prescott Joule.

In his essay on 'Circles' Mr. Emerson, if I remember rightly,
pictured intellectual progress as rhythmic. At a given moment
k