ade on Mount Vesuvius early in 1846,[943] were
repeated with like result by Zantedeschi at Venice four years later. A
rough measure of the intensity of those effects was arrived at by Piazzi
Smyth at Guajara, on the Peak of Teneriffe, in 1856. At a distance of
fifteen feet from the thermomultiplier, a Price's candle was found to
radiate just twice as much heat as the full moon.[944] Then, after
thirteen years, in 1869-72, an exact and extensive series of
observations on the subject were made by the present Earl of Rosse. The
lunar radiations, from the first to the last quarter, displayed, when
concentrated with the Parsonstown three-foot mirror, appreciable thermal
energy, increasing with the phase, and largely due to "dark heat,"
distinguished from the quicker-vibrating sort by inability to traverse a
plate of glass. This was supposed to indicate an actual heating of the
surface, during the long lunar day of 300 hours, to about 500 deg. F.[945]
(corrected later to 197 deg.),[946] the moon thus acting as a direct
radiator no less than as a reflector of heat. But the conclusion was
very imperfectly borne out by Dr. Boeddicker's observations with the
same instrument and apparatus during the total lunar eclipse of October
4, 1884.[947] This initial opportunity of measuring the heat phases of
an eclipsed moon was used with the remarkable result of showing that the
heat disappeared almost completely, though not quite simultaneously,
with the light. Confirmatory evidence of the extraordinary promptitude
with which our satellite parts with heat already to some extent
appropriated, was afforded by Professor Langley's bolometric
observations at Allegheny of the partial eclipse of September 23,
1885.[948] Yet it is certain that the moon sends us a perceptible
quantity of heat _on its own account_, besides simply throwing back
solar radiations. For in February, 1885, Professor Langley succeeded,
after many fruitless attempts, in getting measures of a "lunar
heat-spectrum." The incredible delicacy of the operation may be judged
of from the statement that the sum-total of the thermal energy dispersed
by his rock-salt prisms was insufficient to raise a thermometer fully
exposed to it one-thousandth of a degree Centigrade! The singular fact
was, however, elicited that this almost evanescent spectrum is made up
of two superposed spectra, one due to reflection, the other, with a
maximum far down in the infra-red, to radiation.[949] The co