ars, this was all
that could be made out, though the experiment was often repeated.

It was not until 1831 that Melloni, with his newly-invented
"thermopile," [1] succeeded in making the lunar heat sensible; and in
1835, taking his apparatus to the top of Vesuvius, he obtained not
only perceptible, but measurable, results, getting a deviation of four
or five divisions of his galvanometer.

[Footnote 1: Probably most of our readers know that the
thermopile consists of a number of little bars of two different
metals, connected in pairs, and having the ends joined in a
conducting circuit with a galvanometer. If, now, one set of the
junctures is heated more than the other set, a current of
electricity will be generated, which will affect the
galvanometer. The bars are usually made of bismuth and antimony
though iron and German silver answer pretty well. They are
commonly about half or three-quarters of an inch long, and about
half as large as an ordinary match. The "pile" is made of from
fifty to a hundred such bars packed closely, but insulated by
thin strips of mica, except just at the soldered junctions. With
an instrument of this kind and a very delicate galvanometer,
Professor Henry found that the heat from a person's face could be
perceived at a distance of several hundred feet. There is
however, some doubt whether he was not mistaken in respect to
this extreme sensitiveness.]

Others repeated the experiment several times between this time and
1856, with more or less success; but, so far as I know, the first
quantitative result was that obtained in 1856 by Piazzi Smyth during
his Teneriffe expedition. On the top of the mountain, at an elevation
of ten thousand feet, he found that the moon's rays affected his
thermopile to the same extent as a standard candle ten feet away.
Marie Davy has since shown that this corresponds to a heating effect
of about 1/1300 of a Centigrade degree.

The subject was resumed in 1868 by Lord Rosse in Ireland; and a long
series of observations, running through several years, was made by the
aid of his three-foot reflector (not the great _six_-foot instrument,
which is too unwieldy for such work). The results of his work have,
until very recently, been accepted as authoritative. It should be
mentioned that, at about the same time, observations were also made at
Paris by Marie Davy and Martin; but they are generally looked upon
merely a