m us, with such a motion as
would ever maintain that position, thus securing full moon from sunset
to sunrise, without possibility of eclipse. But Lionville demonstrates
that "if the moon had occupied at the beginning the position assigned
her, by the illustrious author of the _Mecanique Celeste_, she could not
have maintained it but a very short time."[340] In short, La Place's
hypothetical calculations generally have proved erroneous when applied
to any existing facts; and we have no reason to attach more value to his
nebular theory calculations.

The sun is the principal orb of our system, and by far the most
conspicuous, and the most observed of all observers, astronomers
included. But we have seen already how contradictory their measurements
of his distance, and their observations of the influence of his spots.
Far more conflicting are the theories as to his constitution, of which
indeed we may truly say very little was known before the application of
photography and the spectroscope to heliography within the last seven
years. One astronomer fixed the period of his rotation at twenty-five
days, fourteen hours, and eight minutes; another at twenty-six days,
forty-six minutes; another at twenty-four days, twenty-eight
minutes.[341]

In regard to the sun's heat, a matter fundamental to the nebular
theory, the calculations differ widely, and some of them must be grossly
erroneous. M. Vicaire called the attention of the French Academy, at a
recent meeting, to this unsatisfactory condition of science. Father
Secchi estimates it at eighteen million Fahrenheit; while Pouillet says
it ranges from two thousand six hundred and sixty-two to three thousand
two hundred and one; and others range from two hundred thousand
downward. The most singular thing is that these results are derived from
observations or radiations made by apparatus identical in
principle.[342] But Waterston calculates the temperature of the solar
surface at above ten, and probably twelve million Fahrenheit.[343]

Now what feeds these enormous fires? The old opinion of astronomy, that
the sun was a mass of fire, was assailed by Sir Wm. Herschel, who
maintained that it was in the condition of a perpetual magnetic storm.
This notion was altered into the belief of a central dark body,
surrounded by a stratum of clouds, outside of which is a photosphere of
light and heat; which some made one thousand five hundred miles in
depth, others four thousand. Outside o