No. XIII.
THUNDER--LIGHTNING--AURORA BOREALIS--EARTHQUAKES--EBBING AND FLOWING OF
THE SEA--THE LOADSTONE AND AMBER--ELECTRICITY--RIVERS.
Some of the moderns have assigned the cause of _Thunder_ to inflamed
exhalations, rending the clouds wherein they are confined; others, to
the shock 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.
This is the explanation of Descartes, and had but few followers; the
former had more, being that of the Newtonians. For a third theory, which
makes the matter productive of thunder the same with that of
electricity, its author, Dr. Franklin, is in no part indebted to the
ancients.
The notion of Descartes entirely 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.” Anaxagoras refers it to the same
cause.
All the other passages, which occur in such abundance among the
ancients, respecting thunder, contain in them the reasonings of the
Newtonians, sometimes combining the notions of Descartes.
Leucippus, and the Eleatic sect, 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.
According to the stoics, 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. 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 rencontre: he
added, that these effects were coincident; our perception of the
lightning before the thunder-clap being entirely owing to our sight’s
being quicker than our hearing.
In short, Aristophanes, in his comedy of the “Clouds,” introducing
Socrates as satisfying the curiosity of one of his disciples as to the
cause of thunder, makes him assign it to the action of the compressed
air in a cloud, which dilating itself bursts it, and, violently
agitating the exterior air, sets itself on fire, and by the rapidity of
its progress occasions all that noise.
The _Aurora Borealis_ was also observed by the ancients, as may be seen
in Aristotle, Pliny, Seneca, and other writers, who conjectured
differently its cause.
The Cartesians, Newtonians, and other able moderns, ascribe
_Earthquakes_ to the earth’s being filled with cavities of a vast
extent, containing in them an immense quantity of thick exhalations,
resembling the smoke of an extinguished candle, which being easily
inflammable, and by their agitation catching fire, rarefy and heat the
central and condensed air of the cavern to such a degree, that finding
no vent, it bursts its enclosements; and, in doing this, shakes the
surrounding earth all around with dreadful percussions, producing all
the other effects which naturally follow.
Aristotle and Seneca assigned these dreadful events to the same cause.
The former says, 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.
Seneca is so 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 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.
Of all the solutions of the _Ebbing and Flowing of the Sea_, the most
simple and ingenious, though afterwards found by observation to be
inadequate, is that of Descartes, who supposes a vortex of subtile
matter, of an elliptic form, to invest our globe, and compress it on all
sides. The moon, according to this philosopher, is immerged in this
elliptic vortex, and when at its greatest elongation from the earth, it
makes less impression upon the circumambient ethereal matter; but when
it comes to the narrowest part of the ellipse, gives such an impulse to
the atmosphere, as puts the whole ocean in agitation. He supports his
system by this remark, that the ebbing and flowing of the sea generally
coincides with the irregularity of the moon’s course.
The opinion of Kepler and Newton is more conformable to observation, and
is founded on this hypothesis--that the moon attracts the waters of the
sea, diminishing the weight of 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 the sun and moon.
The Cartesian method of solution has been indicated by Pytheas
Massiliensis, who observes, that the tides, in their increase and
decrease, follow the irregular course of the moon; and by Seleucus of
Erythrea, the mathematician, who ascribing to the earth a rotation about
its axis, imputes the cause of tides to the activity of the earth’s
vortex, in conjunction with that of the moon.
Pliny’s account has more affinity to that of sir Isaac Newton. The great
naturalist of the ancients maintained, that “the sun and moon had a
reciprocal share in causing the tides:” and after a course of
observations for many years, he remarked, that “the moon acted most
forcibly upon the waters when it was 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 action of celestial causes, and the
discernible result of them on earth.” He remarked also, that the waters,
which are naturally inert, do not 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.
There are few things which have more engaged the attention of
naturalists, and with less success, than the wonderful properties of the
_Loadstone_. Almost all have agreed in affirming that there are
corpuscles of a peculiar form and energy that continually circulate
around and through the loadstone, and that a vortex of the same matter
circulates around and through the earth. Upon these suppositions
Descartes and others have advanced, that the loadstone has 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 impulse 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.
All this the ancients had said before. The 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.
Descartes’s idea of his explanation was doubtless derived from
Lucretius, who admitted, that there was a “vortex of corpuscles, or
magnetic matter, which, continually circulating around the loadstone,
repelled the intervening air betwixt itself and the iron. The air thus
repelled, the intervening space 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, that “amber attracts
none of those things that are brought to it, any more than the
loadstone, but emits a matter, which reflects the circumambient air, and
thereby forms a void. The 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, to wit, “why the vortex which circulates around
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.”
Certain authors report, that the properties of the loadstone,
particularly its tendency towards the north pole, enabled the ancients
to undertake long voyages; and they pretend, that the Egyptians,
Phœnicians, and Carthaginians, employed the compass to guide them in
their naval excursions; though afterwards they lost the use of it, just
as they did of dying purple,[462] and of embroidering, and of composing
bricks, and a cement able to resist the force of all weathers; arts,
without all doubt, formerly well known to them. Pineda and Kircher
affirm likewise, that Solomon knew the use of the compass, and that his
subjects steered their course by it in sailing to the land of Ophir.
There is also a passage of Plautus[463] produced, wherein it is alleged
he speaks of the compass. There is not however a single passage in the
ancients that directly supports these pretensions.[464]
It is scarcely credible, that the real cause of _Electricity_ was known
to the ancients, and yet there are indications of it in the work of
Timæus Locrensis, concerning the soul of the world.
The moderns are also divided in their sentiments, as to how it happens
that _Rivers_, continually flowing into the sea, do not swell the mass
of waters, so as to make it overflow its banks. One of the solutions of
this difficulty is, that rivers return again to their source by
subterraneous passages or canals; and that there is, between the sea and
the springs of rivers, a circulation analogous to that of blood in the
human body. This solution, however, is the same as Seneca’s, who
accounts for their not overflowing the bed of the ocean, by imagining
secret passages, which reconduct them to their springs; and because, at
their springs, they retain nothing of that brackishness which they
carried with them from the sea, he supposes they are filtrated in their
circuit through winding paths, and layers of every soil, so that they
must needs return to their source as pure and sweet as they departed
thence.
[462] We may with exactness determine what the true colour was of the
purple of the ancients, by attending to two passages of Pliny, wherein
he says, that the whole aim of the Tyrians and Phœnicians, in bringing
their purple to the utmost perfection, was to render it in colour as
like as possible to the oriental amethyst. Plin. Hist. Natur. lib. ix.
c. 38 & 41, et lib. xxxvii. c. 9.
[463]
Hùc secundus ventus nunc est; cape modò Vorsoriam,
Stasime; cape Vorsoriam, recipe te ad Herum.
[464] With respect to what was known to the ancients, and of which we
still are ignorant, recourse may be had to Pancirolus _de rebus
Deperditis_, particularly to his first book, chap. i. 35, 36, 39,
respecting the colour of purple, the ductility of glass and the
effects of the ancient music. See especially Dion. Cassius’s History,
in Tiber. lib. lvii. p. 617. E. Plin. lib. xxxvi. c. 26, &c. Isidor.
de Originib. lib. xvi. c. 15, respecting the ductility of glass.
* * * * *