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CHAPTER IX
OF THUNDER AND EARTHQUAKES; OF THE VIRTUE OF TILE MAGNET;
OF THE EBBING AND FLOWING OF THE SEA; AND OF THE SOURCE OF RIVERS.
1. I go on to some articles of natural philosophy,
where I shall endeavour to show the conformity there is between
the ancients, and some of our most celebrated philosophers. It
is evident, that the causes of thunder, earthquakes, the attractive
force of the loadstone, the ebbing and flowing of the sea, and
the return of rivers to their source were not hid from the former:
nor was it their fault, that the sentiments they so long ago held
on these subjects, were either not adopted, or not till very lately.
It ought not to be objected here, that the diversity of opinions
among them was not so great, that it was difficult to determine
which to choose: unless at the same time, it be acknowledged,
that the same holds true with respect to the equal variety that
reigns at present among us. It is not long ago, that two or three
different sets of notions were raised up against those of Sir
Isaac Newton, respecting colours, but that did not impede the
triumph of his system, nor strip him of the glory of having proposed,
what, beyond all others, was most just and solid.
2. The moderns are divided into two opinions
as to what occasions thunder; some of them assigning the cause
of it to inflamed exhalations, rending the clouds wherein they
are confined; others ascribing it to the shock that happens between
two or more clouds, when those that are higher and more condensed,
fall upon those that are lower, with so much force as suddenly
to expel the Intermediate air, which vigorously expanding itself,
in order to occupy its former space, puts all the exterior air
in commotion, producing those reiterated claps which we call thunder.
I stop not to examine a third theory, which makes the matter productive
of thunder, the same with that which is the cause of electricity;
for though it be the most probable of any, yet the truth of it
is still contested.
3. Of those two sentiments of the ancients,
which have been adopt ed by our moderns, the latter belongs to
Aristotle, who says, that thunder is caused by a dry exhalation,
which falling upon a humid cloud, and violently endeavouring to
force a passage for itself, produces the peals which we hear.
And Anaxagoras refers it to the same cause. All the other passages,
which occur in such abundance among the ancients, respecting the
formation of thunder, evidently con-thin the reasonings of the
Newtonians, and sometimes join together the two sentiments which
divide the moderns.
4. Leucippus held, that thunder proceeded
from a fiery exhalation, which enclosed in a cloud, burst it asunder,
and forced its way through. Democritus asserts, that it is the
effect of a mingled collection of various volatile particles,
which impel downwards the cloud which contains them, till by the
rapidity of their motion, they set themselves and it on fire.
Seneca ascribes it to a dry sulphureous exhalation arising out
of the earth, which he calls the aliment of lightning; and which,
becoming more and more subtilized in its ascent, at last takes
fire in the air, and produces a violent eruption.
5. The Stoics distinguish two things in thunder,
the lightning and the noise. According to them, thunder was occasioned
by the shock of clouds ; and lightning was the combustion of the
volatile parts of the cloud, set on. fire by the shock: and Chrysippus
taught, that lightning was the result of clouds being set on fire
by winds, which dashed them one against another; and that thunder
was the noise produced by that re-encounter: he added, that these
effects were coincident ; our perception of the lightning before
the thunder-clap, being entirely owing to our sight being quicker
than our hearing.
6. There is but one opinion respecting the
cause of earthquakes, which deserves any notice; and it is that
of the Cartesians, Newtonians, and all our other able naturalists,
They ascribe it to the earth’s being filled with cavities
of a vast extent, containing in them an immense quantity of thick
exhalations, of a fuliginous substance, resembling the smoke
of an extinguished candle, which being easily inflammable, arid
by their agitation catching fire, rarely and heat the central
condensed air of the cavern to such a degree, that finding no
vent to issue it, it bursts its enclosements ; and in doing this,
shakes the earth all around with dreadful percussions, producing
all the other effects which naturally follow.
7. The same reason is given by Aristotle
and Seneca, in assigning the cause of such dreadful events. The
former, after refuting those who ascribed earthquakes to the earth
itself, or the water it contains, subjoins his own opinion, “that
they were occasioned by the efforts of the internal air in dislodging
itself from the bowels of the earth ;“ and he observes, that
“on the approach of an earthquake, the weather is generally
serene, because that sort of air which occasions commotions in
the atmosphere, is at that time pent up in the entrails of the
earth.”
8. Seneca is still more precise ; we might
take him for a naturalist of the present times. He supposes, that
“the earth hides in its bosom many subterraneous fires which
uniting their flames, necessarily put into fervid motion the congregated
vapours of its cells, which finding no immediate outlet, exert
their utmost powers, till at last they force a way through whatever
opposes them.” He says also, that if the vapours be too weak
to burst the barriers which retain them, all their efforts end
in weak shocks, and hollow murmurs, without any fatal consequence..
9. Of all the solutions that ever were attempted
to be given of the ebbing and flowing of the sea, the most simple
and ingenious, is, that of Kepler and Sir Isaac Newton. It is
founded on this hypothesis, that the moon attracts the waters
of the sea, diminishing the weight of all those parts of it over
whose zenith it comes, and increasing the weight of the collateral
parts, so that the parts directly opposite to the moon, and under
it in the same hemisphere, must become more elevated than the
rest. According to this system, the action of the sun concurs
with that of the moon, in occasioning the tides; which are higher
or lower respectively, according to the situation of those two
luminaries, which, when in conjunction, act in concert, raising
the tides to the greatest height; and when in opposition, produce
nearly the same effect, in swelling the waters of the opposite
hemispheres: but when in quadrature, suspend each other’s
force, so as to act only by the difference of their powers: and
thus the tides vary, according to the different positions of those
luminaries.
*would be more philosophical to say, the moon attracts
the atmosphere, and the waters naturally conform themselves to
its figure, or to its various degree! of pressure. See Note to
page 451, vol. I.
10. Pliny’s
account agrees with this. That great naturalist. maintained, “
that the sun and moon had a reciprocal share in causing the tides
;“ and after a course of observations for many years, remarks
that the moon acted most forcibly upon the waters, when it nearest
to the earth, but that the effect was not immediately perceived
by us, but at such an interval as may well take place between
the actions of celestial causes, and the discernible result of
them on earth. He remarked also, that the waters, which are naturally
inert, do swell up immediately upon the conjunction of the sun
and moon; but having gradually admitted the impulse, and begun
to raise themselves continue in that elevation, even after the
conjunction is over.
11. There
are few things which have more engaged the attention of naturalists,
and with less success, than the wonderful properties the loadstone.
At all times men have hazarded a variety of conjectures, to account
for the curious effects of it. Almost all have agreed in assigning
this as a principal reason, that there are corpuscles of a peculiar
form and energy, that continually circulate around and through,
the loadstone, and a vortex of the same matter, circulating around
and through the earth. Upon these suppositions, the modern philosophers
have advanced, that the loadstone hath two poles, similar to those
of the earth ; and that the magnetic matter which issues at one
of the poles, and circulates around to enter at the other, occasions
that hath pulse which brings iron to the loadstone, whose, small
corpuscles have an analogy to the pores of iron, fitting them
to lay hold of it, but not of other bodies. This is almost all
that hath been reasonably advanced with respect to the virtue
of the magnet, and all this the ancients had said before.
12. This
impulsive force, which joins iron to the loadstone, and other
things to amber, was known to Plato ; though he would not call
it attraction, as allowing no such cause in nature. This philosopher
called the magnet, the stone of Hercules, because it subdued iron,
which conquers every thing. Lucretius also knew what caused this
property in the loadstone, and without doubt furnished Descartes
with his explanation. He admitted, that there was a “vortex
of corpuscles, or magnetic matter, which continually circulating
around the load-stone, repelled the intervening air betwixt itself
and the iron. The air thus repelled, the intervening space,”
says that philosopher,.” became a vacuum ; and the iron,
finding no resistance, approached with an impulsive force, pushed
on by the air behind it.” Plutarch likewise is of the same
opinion. He says, “amber attracts none of those things that
are brought to it, any more than the loadstone. That stone emits
a matter, which reflects the circumambient air, and thereby forms
a void. That expelled air puts in motion the air before it, which
making a circle returns to the void space, driving before it,
towards the loadstone, the iron which it meets in its way.”
He then proposes a difficulty, “ why the vortex which circulates
round the loadstone, does not make its way to wood or stone, as
well as iron ?“ He answers, like Descartes, that the pores
of iron have an analogy to the particles of the vortex circulating
about the loadstone, which yields them such access as they can
find in no other bodies, whose pores are differently formed.
13. It is scarce credible, that the real
cause of electricity was known to the ancients, though there be
indications of it in the work of Timoeus Locrensis, concerning
the soul of the world, a respectable monument of ancient philosophy.
It is true, that modern naturalists themselves are divided on
this point, not indeed with respect to the general cause of electricity,
hut with regard to the causes of the different directions of
the electric matter. They do not indeed say wherein the essence
of this matter consists; they only define it by its properties,
and explain it by its effects; yet all own, that it is “a
very subtile fluid,” residing around electric bodies, which
upon being put into motion by the friction of those bodies, or
any other cause, forcibly rushes into them, carrying along with
it all the minute things contained in its vortex, and producing
all the other effects of electricity which we perceive : now
this is precisely what Tioemus says of it, in giving the reason
of amber’s attracting bodies ; “ this happens,”
says he, “ because there issues from the amber a subtile
matter, by which it draws other bodies to itself.”
14. The moderns are also divided in their
sentiments, how it comes to pass, that rivers continually flowing
into the sea, do not swell its mass of waters, so as to make it
overflow its banks. One of the chief solutions of this difficulty
is, that rivers return again to their source by subterraneons
passages, which nature hath formed for that purpose; there being
between the sea and the springs of rivers, a circulation analogous
to that of blood in the human body. This explanation of the origin
of rivers, and the comparison respecting their circulation, is
taken from Seneca: who accounts not only for their not overflowing
the bed of the ocean, by the secret passages formed for them by
nature to reconduct them to their springs; but assigns this reason
why, at their springs, they retain nothing of that brackishness,
which they carried with them from the sea; because, says he, they
are completely filtrated in that extensive circuit they make under
ground, through winding paths of all dimensions, and through layers
of every soil ; so that they must needs return to their source,
as pure and sweet as they departed thence.
Chapter 10
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