But we owe to him the elements of our knowledge of the Sun's physical constitution. He swept aside the erroneous theories and conjectures which had previously prevailed, and guided the astronomical inquirer into the right path. He convinced himself, by long and patient researches, that the luminous envelope of the great "orb of day" was neither a liquid nor an elastic fluid; that it was in certain respects analogous to the clouds which wreathe our mountain-summits and fertilize our plains; that it floated in the solar atmosphere. Thence he came to the conclusion that the Sun has two atmospheres, endowed with motions quite independent of each other. An elastic fluid, now known as the _photosphere_, is in course of continual formation on the dark rugged surface of the solar mass; and rising, on account of its specific lightness, it forms the _pores_ in the stratum of reflecting clouds; then, combining with other gases, it produces the irregularities or furrows in the luminous cloud-region. When the ascending currents are powerful, they create those appearances which astronomers designate the _nuclei_, the _penumbrae_, the _faculae_.
Such was Herschel's explanation of the mode of formation of the solar spots; and allowing it to be well-founded, we must expect to find--what is, indeed, the case--that the Sun does not always and regularly pour forth equal quantities of light and heat. It is true that Herschel's hypothesis has been modified by later astronomers; but his is the credit of having directed them into the right course of inquiry and observation.
The physical constitution of the Moon was a subject which also engaged the attention of our indefatigable enthusiast. As early as 1780 he attempted the measurement of the lunar mountains, and came to the conclusion that few of them exceeded 2600 feet in height. Later research, however, has proved these figures to be inadequate. Next he addressed himself to a study of the lunar volcanoes, three of which he declared to be in a state of ignition; two of them apparently on the decline, the third still active. He was so convinced of the reality of the phenomenon, that on the 20th of April 1787 he wrote:--"The volcano burns with greater violence than it did last night." The real diameter of the volcanic light he estimated at 16,400 feet. Its intensity he described as superior to that of the nucleus of a comet then flashing across our system. The objects situated near the crater were fully illuminated by the glare of its burning matter.
