whirlings, the parts of air acquiring a circular motion, and receding
from the middle of the circle by a centrifugal force, and leaving there
a vacancy; if descending, greatest above and lessening downward; if
ascending, greatest below and lessening upward; like a speaking-trumpet
standing its big end on the ground.
When the air descends with a violence in some places, it may rise with
equal violence in others, and form both kinds of whirlwinds.
The air, in its whirling motion, receding every way from the centre or
axis of the trumpet, leaves there a _vacuum_, which cannot be filled
through the sides, the whirling air, as an arch, preventing; it must
then press in at the open ends.
The greatest pressure inward must be at the lower end, the greatest
weight of the surrounding atmosphere being there. The air, entering,
rises within, and carries up dust, leaves, and even heavier bodies that
happen in its way, as the eddy or whirl passes over land.
If it passes over water, the weight of the surrounding atmosphere forces
up the water into the vacuity, part of which, by degrees, joins with the
whirling air, and, adding weight and receiving accelerated motion,
recedes farther from the centre or axis of the trump as the pressure
lessens; and at last, as the trump widens, is broken into small
particles, and so united with air as to be supported by it, and become
black clouds at the top of the trump.
Thus these eddies may be whirlwinds at land, water-spouts at sea. A body
of water so raised may be suddenly let fall, when the motion, &c., has
not strength to support it, or the whirling arch is broken so as to
admit the air: falling in the sea, it is harmless unless ships happen
under it; and if in the progressive motion of the whirl it has moved
from the sea over the land, and then breaks, sudden, violent, and
mischievous torrents are the consequences.
* * * * *
_To Dr. Perkins._
_Water-spouts and Whirlwinds compared._--Read at the Royal Society,
June 24, 1753.
Philadelphia, Feb. 4, 1753.
I ought to have written to you long since, in answer to yours of October
16, concerning the water-spout; but business partly, and partly a desire
of procuring farther information by inquiry among my seafaring
acquaintance, induced me to postpone writing, from time to time, till I
am almost ashamed to resume the subject, not knowing but you may have
forgot what has been said upon it.
Nothing certainly can be more improving to a searcher into nature than
objections judiciously made to his opinion, taken up, perhaps, too
hastily: for such objections oblige him to restudy the point, consider
every circumstance carefully, compare facts, make experiments, weigh
arguments, and be slow in drawing conclusions. And hence a sure
advantage results; for he either confirms a truth before too slightly
supported, or discovers an error, and receives instruction from the
objector.
In this view I consider the objections and remarks you sent me, and
thank you for them sincerely; but, how much soever my inclinations lead
me to philosophical inquiries, I am so engaged in business, public and
private, that those more pleasing pursuits are frequently interrupted,
and the chain of thought necessary to be closely continued in such
disquisitions is so broken and disjointed, that it is with difficulty I
satisfy myself in any of them; and I am now not much nearer a conclusion
in this matter of the spout than when I first read your letter.
Yet, hoping we may, in time, sift out the truth between us, I will send
you my present thoughts, with some observations on your reasons on the
accounts in the _Transactions_, and on other relations I have met with.
Perhaps, while I am writing, some new light may strike me, for I shall
now be obliged to consider the subject with a little more attention.
I agree with you, that, by means of a vacuum in a whirlwind, water
cannot be supposed to rise in large masses to the region of the clouds;
for the pressure of the surrounding atmosphere could not force it up in
a continued body or column to a much greater height than thirty feet.
But if there really is a vacuum in the centre, or near the axis of
whirlwinds, then, I think, water may rise in such vacuum to that height,
or to a less height, as the vacuum may be less perfect.
I had not read Stuart's account, in the _Transactions_, for many years
before the receipt of your letter, and had quite forgot it; but now, on
viewing his draughts and considering his descriptions, I think they seem
to favour _my hypothesis_; for he describes and draws columns of water
of various heights, terminating abruptly at the top, exactly as water
would do when forced up by the pressure of the atmosphere into an
exhausted tube.
I must, however, no longer call it _my hypothesis_, since I find Stuart
had the same thought, though somewhat obscurely expressed, where he says
"he imagines this phenomenon may be solved by suction (improperly so
called) or rather pulsion, as in the application of a cupping-glass to
the flesh, the air being first voided by the kindled flax."
In my paper, I supposed a whirlwind and a spout to be the same thing,
and to proceed from the same cause; the only difference between them
being that the one passes over the land, the other over water. I find
also in the _Transactions_, that M. de la Pryme was of the same opinion;
for he there describes two spouts, as he calls them, which were seen at
different times, at Hatfield, in Yorkshire, whose appearances in the air
were the same with those of the spouts at sea, and effects the same with
those of real whirlwinds.
Whirlwinds have generally a progressive as well as a circular motion; so
had what is called the spout at Topsham, as described in the
Philosophical Transactions, which also appears, by its effects
described, to have been a real whirlwind. Water-spouts have, also, a
progressive motion; this is sometimes greater and sometimes less; in
some violent, in others barely perceivable. The whirlwind at Warrington
continued long in Acrement Close.
Whirlwinds generally arise after calms and great heats: the same is
observed of water-spouts, which are, therefore, most frequent in the
warm latitudes. The spout that happened in cold weather, in the Downs,
described by Mr. Gordon in the _Transactions_, was, for that reason,
thought extraordinary; but he remarks withal, that the weather, though
cold when the spout appeared, was soon after much colder: as we find it
commonly less warm after a whirlwind.
You agree that the wind blows every way towards a whirlwind from a large
space round. An intelligent whaleman of Nantucket informed me that three
of their vessels, which were out in search of whales, happening to be
becalmed, lay in sight of each other, at about a league distance, if I
remember right, nearly forming a triangle: after some time, a
water-spout appeared near the middle of the triangle, when a brisk
breeze of wind sprung up, and every vessel made sail; and then it
appeared to them all, by the setting of the sails and the course each
vessel stood, that the spout was to the leeward of every one of them;
and they all declared it to have been so when they happened afterward in
company, and came to confer about it. So that in this particular,
likewise, whirlwinds and water-spouts agree.
But if that which appears a water-spout at sea does sometimes, in its
progressive motion, meet with and pass over land, and there produce all
the phenomena and effects of a whirlwind, it should thence seem still
more evident that a whirlwind and a spout are the same. I send you,
herewith, a letter from an ingenious physician of my acquaintance, which
gives one instance of this, that fell within his observation.
A fluid, moving from all points horizontally towards a centre, must, at
that centre, either ascend or descend. Water being in a tub, if a hole
be opened in the middle of the bottom, will flow from all sides to the
centre, and there descend in a whirl. But air flowing on and near the
surface of land or water, from all sides towards a centre, must at that
centre ascend, the land or water hindering its descent.
If these concentring currents of air be in the upper region, they may,
indeed, descend in the spout or whirlwind; but then, when the united
current reached the earth or water, it would spread, and, probably, blow
every way from the centre. There may be whirlwinds of both kinds, but
from the commonly observed effects I suspect the rising one to be the
most common: when the upper air descends, it is, perhaps, in a greater
body, extending wider, as in our thunder-gusts, and without much
whirling; and, when air descends in a spout or whirlwind, I should
rather expect it would press the roof of a house _inward_, or force _in_
the tiles, shingles, or thatch, force a boat down into the water, or a
piece of timber into the earth, than that it would lift them up and
carry them away.
It has so happened that I have not met with any accounts of spouts that
certainly descended; I suspect they are not frequent. Please to
communicate those you mention. The apparent dropping of a pipe from the
clouds towards the earth or sea, I will endeavour to explain hereafter.
The augmentation of the cloud, which, as I am informed, is generally,
if not always the case, during a spout, seems to show an ascent rather
than a descent of the matter of which such cloud is composed; for a
descending spout, one would expect, should diminish a cloud. I own,
however, that cold air, descending, may, by condensing the vapours in a
lower region, form and increase clouds; which, I think, is generally the
case in our common thunder-gusts, and, therefore, do not lay great
stress on this argument.
Whirlwinds and spouts are not always, though most commonly, in the
daytime. The terrible whirlwind which damaged a great part of Rome, June
11, 1749, happened in the night of that day. The same was supposed to
have been first a spout, for it is said to be beyond doubt that it
gathered in the neighbouring sea, as it could be tracked from Ostia to
Rome. I find this in Père Boschovich's account of it, as abridged in the
Monthly Review for December, 1750.
In that account, the whirlwind is said to have appeared as a very black,
long, and lofty cloud, discoverable, notwithstanding the darkness of the
night, by its continually lightning or emitting flashes on all sides,
pushing along with a surprising swiftness, and within three or four feet
of the ground. Its general effects on houses were stripping off the
roofs, blowing away chimneys, breaking doors and windows, _forcing up
the floors, and unpaving the rooms_ (some of these effects seem to agree
well with a supposed vacuum in the centre of the whirlwind), and the
very rafters of the houses were broken and dispersed, and even hurled
against houses at a considerable distance, &c.
It seems, by an expression of Père Boschovich's, as if the wind blew
from all sides towards the whirlwind; for, having carefully observed its
effects, he concludes of all whirlwinds, "that their motion is circular,
and their action attractive."
He observes on a number of histories of whirlwinds, &c., "that a common
effect of them is to carry up into the air tiles, stones, and animals
themselves, which happen to be in their course, and all kinds of bodies
unexceptionably, throwing them to a considerable distance with great
impetuosity."
Such effects seem to show a rising current of air.
I will endeavour to explain my conceptions of this matter by figures,
representing a plan and an elevation of a spout or whirlwind.
I would only first beg to be allowed two or three positions mentioned in
my former paper.
1. That the lower region of air is often more heated, and so more
rarefied, than the upper; consequently, specifically lighter. The
coldness of the upper region is manifested by the hail which sometimes
falls from it in a hot day.
2. That heated air may be very moist, and yet the moisture so equally
diffused and rarefied as not to be visible till colder air mixes with
it, when it condenses and becomes visible. Thus our breath, invisible in
summer, becomes visible in winter.
Now let us suppose a tract of land or sea, of perhaps sixty miles
square, unscreened by clouds and unfanned by winds during great part of
a summer's day, or, it may be, for several days successively, till it is
violently heated, together with the lower region of air in contact with
it, so that the said lower air becomes specifically lighter than the
superincumbent higher region of the atmosphere in which the clouds
commonly float: let us suppose, also, that the air surrounding this
tract has not been so much heated during those days, and, therefore,
remains heavier. The consequence of this should be, as I conceive, that
the heated lighter air, being pressed on all sides, must ascend, and the
heavier descend; and as this rising cannot be in all parts, or the whole
area of the tract at once, for that would leave too extensive a vacuum,
the rising will begin precisely in that column that happens to be the
lightest or most rarefied; and the warm air will flow horizontally from
all points to this column, where the several currents meeting, and
joining to rise, a whirl is naturally formed, in the same manner as a
whirl is formed in the tub of water, by the descending fluid flowing
from all sides of the tub to the hole in the centre.
And as the several currents arrive at this central rising column with a
considerable degree of horizontal motion, they cannot suddenly change it
to a vertical motion; therefore, as they gradually, in approaching the
whirl, decline from right curved or circular lines, so, having joined
the whirl, they _ascend_ by a spiral motion, in the same manner as the
water _descends_ spirally through the hole in the tub before mentioned.
Lastly, as the lower air, and nearest the surface, is most rarefied by
the heat of the sun, that air is most acted on by the pressure of the
surrounding cold and heavy air, which is to take its place;
consequently, its motion towards the whirl is swiftest, and so the force
of the lower part of the whirl or trump strongest, and the centrifugal
force of its particles greatest; and hence the vacuum round the axis of
the whirl should be greatest near the earth or sea, and be gradually
diminished as it approaches the region of the clouds, till it ends in a
point, as at P, _Fig. 2. in the plate_, forming a long and sharp cone.
In figure 1, which is a plan or groundplat of a whirlwind, the circle V
represents the central vacuum.
Between _a a a a_ and _b b b b_ I suppose a body of air, condensed
strongly by the pressure of the currents moving towards it from all
sides without, and by its centrifugal force from within, moving round
with prodigious swiftness (having, as it were, the entire momenta
of all the currents ----> ----> united in itself), and with
a power equal to its swiftness and density.
It is this whirling body of air between _a a a a_ and _b b b b_ that
rises spirally; by its force it tears buildings to pieces, twists up
great trees by the roots, &c., and, by its spiral motion, raises the
fragments so high, till the pressure of the surrounding and approaching
currents diminishing, can no longer confine them to the circle, or their
own centrifugal force increasing, grows too strong for such pressure,
when they fly off in tangent lines, as stones out of a sling, and fall
on all sides and at great distances.
If it happens at sea, the water under and between _a a a a_ and _b b b
b_ will be violently agitated and driven about, and parts of it raised
with the spiral current, and thrown about so as to form a bushlike
appearance.
This circle is of various diameters, sometimes very large. If the vacuum
passes over water, the water may rise in it in a body or column to near
the height of thirty-two feet. If it passes over houses, it may burst
their windows or walls outward, pluck off the roofs, and pluck up the
floors, by the sudden rarefaction of the air contained within such
buildings; the outward pressure of the atmosphere being suddenly taken
off; so the stopped bottle of air bursts under the exhausted receiver of
the airpump.
Fig. 2 is to represent the elevation of a water-spout, wherein I suppose
P P P to be the cone, at first a vacuum, till W W, the rising column of
water, has filled so much of it. S S S S, the spiral whirl of air,
surrounding the vacuum, and continued higher in a close column after the
vacuum ends in the point P, till it reaches the cool region of the air.
B B, the bush described by Stuart, surrounding the foot of the column of
water.
Now I suppose this whirl of air will at first be as invisible as the
air itself, though reaching, in reality, from the water to the region of
cool air, in which our low summer thunder-clouds commonly float: but
presently it will become visible at its extremities. _At its lower end_,
by the agitation of the water under the whirling part of the circle,
between P and S, forming Stuart's bush, and by the swelling and rising
of the water in the beginning vacuum, which is at first a small, low,
broad cone, whose top gradually rises and sharpens, as the force of the
whirl increases. _At its upper end_ it becomes visible by the warm air
brought up to the cooler region, where its moisture begins to be
condensed into thick vapour by the cold, and is seen first at A, the
highest part, which, being now cooled, condenses what rises next at B,
which condenses that at C, and that condenses what is rising at D, the
cold operating by the contact of the vapours faster in a right line
downward than the vapours can climb in a spiral line upward; they climb,
however, and as by continual addition they grow denser, and,
consequently, their centrifugal force greater, and being risen above the
concentrating currents that compose the whirl, fly off, spread, and form
a cloud.
It seems easy to conceive how, by this successive condensation from
above, the spout appears to drop or descend from the cloud, though the
materials of which it is composed are all the while ascending.
The condensation of the moisture contained in so great a quantity of
warm air as may be supposed to rise in a short time in this prodigiously
rapid whirl, is perhaps sufficient to form a great extent of cloud,
though the spout should be over land, as those at Hatfield; and if the
land happens not to be very dusty, perhaps the lower part of the spout
will scarce become visible at all; though the upper, or what is commonly
called the descending part, be very distinctly seen.
The same may happen at sea, in case the whirl is not violent enough to
make a high vacuum, and raise the column, &c. In such case, the upper
part A B C D only will be visible, and the bush, perhaps, below.
But if the whirl be strong, and there be much dust on the land, and the
column W W be raised from the water, then the lower part becomes visible
and sometimes even united to the upper part. For the dust may be carried
up in the spiral whirl till it reach the region where the vapour is
condensed, and rise with that even to the clouds: and the friction of
the whirling air on the sides of the column W W, may detach great
quantities of its water, break it into drops, and carry them up in the
spiral whirl, mixed with the air; the heavier drops may indeed fly off,
and fall in a shower round the spout; but much of it will be broken into
vapour, yet visible; and thus, in both cases, by dust at land and by
water at sea, the whole tube may be darkened and rendered visible.
As the whirl weakens, the tube may (in appearance) separate in the
middle; the column of water subsiding, and the superior condensed part
drawing up to the cloud. Yet still the tube or whirl of air may remain
entire, the middle only becoming invisible, as not containing visible
matter.
Dr. Stuart says, "It was observable of all the spouts he saw, but more
perceptible of the great one, that, towards the end, it began to appear
like a hollow canal, only black in the borders, but white in the middle;
and though at first it was altogether black and opaque, yet now one
could very distinctly perceive the seawater to fly up along the middle
of this canal, as smoke up a chimney."
And Dr. Mather, describing a whirlwind, says, "A thick dark, small cloud
arose, with a pillar of light in it, of about eight or ten feet
diameter, and passed along the ground in a tract not wider than a
street, horribly tearing up trees by the roots, blowing them up in the
air life feathers, and throwing up stones of great weight to a
considerable height in the air," &c.
These accounts, the one of water-spouts, the other of a whirlwind, seem
in this particular to agree; what one gentleman describes as a tube,
black in the borders and white in the middle, the other calls a black
cloud, with a pillar of light in it; the latter expression has only a
little more of the _marvellous_, but the thing is the same; and it seems
not very difficult to understand. When Dr. Stuart's spouts were full
charged, that is, when the whirling pipe of air was filled between _a a
a a_ and _b b b b_, fig. 1, with quantities of drops, and vapour torn
off from the column W W, fig. 2, the whole was rendered so dark as that
it could not be seen through, nor the spiral ascending motion
discovered; but when the quantity ascending lessened, the pipe became
more transparent, and the ascending motion visible. For, by inspection
of the figure given in the opposite page, respecting a section of our
spout, with the vacuum in the middle, it is plain that if we look at
such a hollow pipe in the direction of the arrows, and suppose opaque
particles to be equally mixed in the space between the two circular
lines, both the part between the arrows _a_ and _b_, and that between
the arrows _c_ and _d_, will appear much darker than that between _b_
and _c_, as there must be many more of those opaque particles in the
line of vision across the sides than across the middle. It is thus that
a hair in a microscope evidently appears to be a pipe, the sides showing
darker than the middle. Dr. Mather's whirl was probably filled with
dust, the sides were very dark, but the vacuum within rendering the
middle more transparent, he calls it a pillar of light.
[Illustration:
Fig. 1
Fig. 2
Fig. 3]
It was in this more transparent part, between _b_ and _c_, that Stuart
could see the spiral motion of the vapours, whose lines on the nearest
and farthest side of the transparent part crossing each other,
represented smoke ascending in a chimney; for the quantity being still
too great in the line of sight through the sides of the tube, the motion
could not be discovered there, and so they represented the solid sides
of the chimney.
When the vapours reach in the pipe from the clouds near to the earth, it
is no wonder now to those who understand electricity, that flashes of
lightning should descend by the spout, as in that of Rome.
But you object, if water may be thus carried into the clouds, why have
we not salt rains? The objection is strong and reasonable, and I know
not whether I can answer it to your satisfaction. I never heard but of
one salt rain, and that was where a spout passed pretty near a ship; so
I suppose it to be only the drops thrown off from the spout by the
centrifugal force (as the birds were at Hatfield), when they had been
carried so high as to be above, or to be too strongly centrifugal for
the pressure of the concurring winds surrounding it: and, indeed, I
believe there can be no other kind of salt rain; for it has pleased the
goodness of God so to order it, that the particles of air will not
attract the particles of salt, though they strongly attract water.