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 momenta of all
the currents -----> -----> -----> -----> united in itself) and with a
power equal to its swiftness and density.
[Illustration: (Water Spouts)
_Plate V._ _Vol. II. page 26._
_Published as the Act directs, April 1, 1806, by Longman, Hurst, Rees
& Orme, Paternoster Row._]
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 encreasing, 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
bush-like 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
outwards, 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 air-pump.
FIG. II. 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 encreases. _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 downwards, than
the vapours themselves can climb in a spiral line upwards; 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 sea-water to fly up
along the middle of this canal, as smoak 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 like feathers, and throwing up stones of great weight to a
considerable height in the air, &c."
[Illustration: (cross-section of a whirlwind)]
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. I.,
with quantities of drops, and vapour torn off from the column W W,
Fig. II., the whole was rendered so dark, as that it could not be
seen thro', 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
in the opposite page, representing 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 shewing 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.
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 smoak 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.
Hence, though all metals, even gold, may be united with air, and
rendered volatile, salt remains fixt in the fire, and no heat can
force it up to any considerable height, or oblige the air to hold
it. Hence, when salt rises, as it will a little way, into air with
water, there is instantly a separation made; the particles of water
adhere to the air, and the particles of salt fall down again, as if
repelled and forced off from the water by some power in the air;
or, as some metals, dissolved in a proper menstruum, will quit the
solvent when other matter approaches, and adhere to that, so the
water quits the salt, and embraces the air; but air will not embrace
the salt, and quit the water, otherwise our rains would indeed be
salt, and every tree and plant on the face of the earth be destroyed,
with all the animals that depend on them for subsistence.----He who
hath proportioned and given proper qualities to all things, was not
unmindful of this. Let us adore HIM with praise and thanksgiving! By
some accounts of seamen, it seems the column of water W W, sometimes
falls suddenly; and if it be, as some say, fifteen or twenty yards
diameter, it must fall with great force, and they may well fear for
their ships. By one account, in the _Transactions_, of a spout that
fell at Colne in Lancashire, one would think the column is sometimes
lifted off from the water, and carried over land, and there let fall
in a body; but this, I suppose, happens rarely.
Stuart describes his spouts as appearing no bigger than a mast, and
sometimes less; but they were seen at a league and a half distance.
I think I formerly read in Dampier, or some other voyager, that a
spout, in its progressive motion, went over a ship becalmed, on the
coast of Guinea, and first threw her down on one side, carrying away
her foremast, then suddenly whipped her up, and threw her down on the
other side, carrying away her mizen-mast, and the whole was over in
an instant. I suppose the first mischief was done by the fore-side of
the whirl, the latter by the hinder-side, their motion being contrary.
I suppose a whirlwind, or spout, may be stationary, when the
concurring winds are equal; but if unequal, the whirl acquires a
progressive motion, in the direction of the strongest pressure.
When the wind that gives the progressive motion becomes stronger
below than above, or above than below, the spout will be bent, and,
the cause ceasing, straiten again.
Your queries, towards the end of your paper, appear judicious,
and worth considering. At present I am not furnished with facts
sufficient to make any pertinent answer to them; and this paper has
already a sufficient quantity of conjecture.
Your manner of accommodating the accounts to your hypothesis of
descending spouts, is, I own, ingenious, and perhaps that hypothesis
may be true. I will consider it farther, but, as yet, I am not
satisfied with it, though hereafter I may be.
Here you have my method of accounting for the principal phenomena,
which I submit to your candid examination.
And as I now seem to have almost written a book, instead of a letter,
you will think it high time I should conclude; which I beg leave to
do, with assuring you, that
I am, Sir, &c.
B. FRANKLIN.
FOOTNOTE:
[6] Perkins. _Editor._
DOCTOR M----[7], TO BENJAMIN FRANKLIN, ESQ. AT PHILADELPHIA.
_Description of a Water-Spout at Antigua._
Read at the Royal Society, June 24, 1756.
_New-Brunswick, November 11, 1752._
SIR,
I am favoured with your letter of the 2d instant, and shall, with
pleasure, comply with your request, in describing (as well as my
memory serves me) the water-spout I saw at Antigua; and shall think
this, or any other service I can do, well repaid, if it contributes
to your satisfaction in so curious a disquisition.
I had often seen water-spouts at a distance, and heard many strange
stories of them, but never knew any thing satisfactory of their
nature or cause, until that which I saw at Antigua; which convinced
me that a water-spout is a whirlwind, which becomes visible in all
its dimensions by the water it carries up with it.
There appeared, not far from the mouth of the harbour of St. John's,
two or three water-spouts, one of which took its course up the
harbour. Its progressive motion was slow and unequal, not in a strait
line, but, as it were, by jerks or starts. When just by the wharf, I
stood about one hundred yards from it. There appeared in the water a
circle of about twenty yards diameter, which, to me, had a dreadful,
though pleasing appearance. The water in this circle was violently
agitated, being whisked about, and carried up into the air with great
rapidity and noise, and reflected a lustre, as if the sun shined
bright on that spot, which was more conspicuous, as there appeared
a dark circle around it. When it made the shore, it carried up with
the same violence shingles, staves[8], large pieces of the roofs of
houses, &c. and one small wooden house it lifted entire from the
foundation on which it stood, and carried it to the distance of
fourteen feet, where it settled without breaking or oversetting;
and, what is remarkable, though the whirlwind moved from west to
east, the house moved from east to west. Two or three negroes and a
white woman, were killed by the fall of timber, which it carried up
into the air and dropped again. After passing through the town, I
believe it was soon dissipated; for, except tearing a large limb from
a tree, and part of the cover of a sugar-work near the town, I do
not remember any farther damage done by it. I conclude, wishing you
success in your enquiry,
And am, &c.
W. M.
FOOTNOTES:
[7] Dr. Mercer. _Editor._
[8] I suppose shingles, staves, timber, and other lumber, might be
lying in quantities on the wharf, for sale, as brought from the
northern colonies. B. F.
DOCTOR ----[9], OF BOSTON, TO BENJAMIN FRANKLIN, ESQ. AT PHILADELPHIA.
_Shooting Stars._
Read at the Royal Society, July 8, 1756.
_Boston, May 14, 1753._
SIR,
I received your letter of April last, and thank you for it. Several
things in it make me at a loss which side the truth lies on, and
determine me to wait for farther evidence.
As to shooting-stars, as they are called, I know very little, and
hardly know what to say. I imagine them to be passes of electric
fire from place to place in the atmosphere, perhaps occasioned by
accidental pressures of a non-electric circumambient fluid, and
so by propulsion, or allicited by the circumstance of a distant
quantity _minus_ electrified, which it shoots to supply, and becomes
apparent by its contracted passage through a non-electric medium.
Electric fire in our globe is always in action, sometimes ascending,
descending, or passing from region to region. I suppose it avoids too
dry air, and therefore we never see these shoots ascend. It always
has freedom enough to pass down unobserved, but, I imagine, not
always so, to pass to distant climes and meridians less stored with
it.
The shoots are sometimes all one way, which, in the last case, they
should be.
Possibly there may be collections of particles in our atmosphere,
which gradually form, by attraction, either similar ones _per se_,
or dissimilar particles, by the intervention of others. But then,
whether they shoot or explode of themselves, or by the approach of
some suitable foreign collection, accidentally brought near by the
usual commotions and interchanges of our atmosphere, especially when
the higher and lower regions intermix, before change of winds and
weather, I leave.
I believe I have now said enough of what I know nothing about. If it
should serve for your amusement, or any way oblige you, it is all
I aim at, and shall, at your desire, be always ready to say what I
think, as I am sure of your candour.
I am, &c.
FOOTNOTE:
[9] Dr. Perkins. _Editor._
_A subsequent Paper from the same._
_Water-Spouts and Whirlwinds._
Read at the Royal Society, July 8, 1756.
Spouts have been generally believed ascents of water from below, to
the region of the clouds, and whirlwinds the means of conveyance.
The world has been very well satisfied with these opinions, and
prejudiced with respect to any observations about them. Men of
learning and capacity have had many opportunities in passing
those regions where these phenomena were most frequent, but seem
industriously to have declined any notice of them, unless to escape
danger, as a matter of mere impertinence in a case so clear and
certain as their nature and manner of operation are taken to be.
Hence it has been very difficult to get any tolerable accounts of
them. None but those they fell near can inform us any thing to be
depended on; three or four such instances follow, where the vessels
were so near, that their crews could not avoid knowing something
remarkable with respect to the matters in question.
Capt. John Wakefield, junior, passing the Straits of Gibraltar, had
one fall by the side of his ship; it came down of a sudden, as they
think, and all agree the descent was certain.
Captain Langstaff, on a voyage to the West Indies, had one come
across the stern of his vessel, and passed away from him. The water
came down in such quantity that the present Captain Melling, who was
then a common sailor at helm, says it almost drowned him, running
into his mouth, nose, ears, &c. and adds, that it tasted perfectly
fresh.
One passed by the side of Captain Howland's ship, so near that it
appeared pretty plain that the water descended from first to last.
Mr. Robert Spring was so near one in the Straits of Malacca, that he
could perceive it to be a small very thick rain.
All these assure me, that there was no wind drawing towards them, nor
have I found any others that have observed such a wind.
It seems plain, by these few instances, that whirlwinds do not always
attend spouts; and that the water really descends in some of them.
But the following consideration, in confirmation of this opinion,
may, perhaps, render it probable that all the spouts are descents.
It seems unlikely that there should be two sorts of spouts, one
ascending and the other descending.
It has not yet been proved that any one spout ever ascended. A
specious appearance is all that can be produced in favour of this;
and those who have been most positive about it, were at more than a
league's distance when they observed, as Stuart and others, if I am
not mistaken. However, I believe it impossible to be certain whether
water ascends or descends at half the distance.
It may not be amiss to consider the places where they happen most.
These are such as are liable to calms from departing winds on both
sides, as on the borders of the equinoctial trade, calms on the
coast of Guinea, in the Straits of Malacca, &c. places where the
under region of the atmosphere is drawn off horizontally. I think
they do not come where the calms are without departing winds; and
I take the reason to be, that such places, and places where winds
blow towards one another, are liable to whirlwinds, or other ascents
of the lower region, which I suppose contrary to spouts. But the
former are liable to descents, which I take to be necessary to their
production. Agreeable to this, it seems reasonable to believe, that
any Mediterranean sea should be more subject to spouts than others.
The sea usually so called is so. The Straits of Malacca is. Some
large gulphs may probably be so, in suitable latitudes; so the Red
Sea, &c. and all for this reason, that the heated lands on each side
draw off the under region of the air, and make the upper descend,
whence sudden and wonderful condensations may take place, and make
these descents.
It seems to me, that the manner of their appearance and procedure,
favour the notion of a descent.
More or less of a cloud, as I am informed, always appears over the
place first; then a spattering on the surface of the water below; and
when this is advanced to a considerable degree, the spout emerges
from the cloud, and descends, and that, if the causes are sufficient,
down to the places of spattering, with a roaring in proportion to the
quantity of the discharge; then it abates, or stops, sometimes more
gradually, sometimes more suddenly.
I must observe a few things on these particulars, to shew how I think
they agree with my hypothesis.
The preceding cloud over the place shews condensation, and,
consequently, tendency downwards, which therefore must naturally
prevent any ascent. Besides that, so far as I can learn, a whirlwind
never comes under a cloud, but in a clear sky.
The spattering may be easily conceived to be caused by a stream of
drops, falling with great force on the place, imagining the spout
to begin so, when a sudden and great condensation happens in a
contracted space, as the Ox-Eye on the coast of Guinea.
The spout appearing to descend from the cloud seems to be, by the
stream of nearly contiguous drops bringing the air into consent,
so as to carry down a quantity of the vapour of the cloud; and the
pointed appearance it makes may be from the descending course being
swiftest in the middle, or centre of the spout: this naturally
drawing the outer parts inward, and the centre to a point; and
that will appear foremost that moves swiftest. The phenomenon of
retiring and advancing, I think may be accounted for, by supposing
the progressive motion to exceed or not equal the consumption of
the vapour by condensation. Or more plainly thus: the descending
vapour which forms the apparent spout, if it be slow in its progress
downwards, is condensed as fast as it advances, and so appears at a
stand; when it is condensed faster than it advances, it appears to
retire; and _vice versa_.
Its duration, and manner of ending, are as the causes, and may vary
by several accidents.
The cloud itself may be so circumstanced as to stop it; as when,
extending wide, it weighs down at a distance round about, while a
small circle at the spout being exonerated by the discharge ascends
and shuts up the passage. A new determination of wind may, perhaps,
stop it too. Places liable to these appearances are very liable to
frequent and sudden alterations of it.
Such accidents as a clap of thunder, firing cannon, &c. may stop
them, and the reason may be, that any shock of this kind may occasion
the particles that are near cohering, immediately to do so; and then
the whole, thus condensed, falls at once (which is what I suppose
is vulgarly called the breaking of the spout) and in the interval,
between this period and that of the next set of particles being ready
to unite, the spout shuts up. So that if this reasoning is just,
these phenomena agree with my hypothesis.