Letters on England [32]
planet; and in this manner shows, from the simple laws of mechanics,
that every celestial globe ought necessarily to be where it is
placed.
His bare principle of the laws of gravitation accounts for all the
apparent inequalities in the course of the celestial globes. The
variations of the moon are a necessary consequence of those laws.
Moreover, the reason is evidently seen why the nodes of the moon
perform their revolutions in nineteen years, and those of the earth
in about twenty-six thousand. The several appearances observed in
the tides are also a very simple effect of this attraction. The
proximity of the moon, when at the full, and when it is new, and its
distance in the quadratures or quarters, combined with the action of
the sun, exhibit a sensible reason why the ocean swells and sinks.
After having shown by his sublime theory the course and inequalities
of the planets, he subjects comets to the same law. The orbit of
these fires (unknown for so great a series of years), which was the
terror of mankind and the rock against which philosophy split,
placed by Aristotle below the moon, and sent back by Descartes above
the sphere of Saturn, is at last placed in its proper seat by Sir
Isaac Newton.
He proves that comets are solid bodies which move in the sphere of
the sun's activity, and that they describe an ellipsis so very
eccentric, and so near to parabolas, that certain comets must take
up above five hundred years in their revolution.
The learned Dr. Halley is of opinion that the comet seen in 1680 is
the same which appeared in Julius Caesar's time. This shows more
than any other that comets are hard, opaque bodies; for it descended
so near to the sun, as to come within a sixth part of the diameter
of this planet from it, and consequently might have contracted a
degree of heat two thousand times stronger than that of red-hot
iron; and would have been soon dispersed in vapour, had it not been
a firm, dense body. The guessing the course of comets began then to
be very much in vogue. The celebrated Bernoulli concluded by his
system that the famous comet of 1680 would appear again the 17th of
May, 1719. Not a single astronomer in Europe went to bed that
night. However, they needed not to have broke their rest, for the
famous comet never appeared. There is at least more cunning, if not
more certainty, in fixing its return to so remote a distance as five
hundred and seventy-five years. As to Mr. Whiston, he affirmed very
seriously that in the time of the Deluge a comet overflowed the
terrestrial globe. And he was so unreasonable as to wonder that
people laughed at him for making such an assertion. The ancients
were almost in the same way of thinking with Mr. Whiston, and
fancied that comets were always the forerunners of some great
calamity which was to befall mankind. Sir Isaac Newton, on the
contrary, suspected that they are very beneficent, and that vapours
exhale from them merely to nourish and vivify the planets, which
imbibe in their course the several particles the sun has detached
from the comets, an opinion which, at least, is more probable than
the former. But this is not all. If this power of gravitation or
attraction acts on all the celestial globes, it acts undoubtedly on
the several parts of these globes. For in case bodies attract one
another in proportion to the quantity of matter contained in them,
it can only be in proportion to the quantity of their parts; and if
this power is found in the whole, it is undoubtedly in the half; in
the quarters in the eighth part, and so on in infinitum.
This is attraction, the great spring by which all Nature is moved.
Sir Isaac Newton, after having demonstrated the existence of this
principle, plainly foresaw that its very name would offend; and,
therefore, this philosopher, in more places than one of his books,
gives the reader some caution about it. He