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.
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