Is it not thus that the surface of this globe is continually heated by such repeated vibrations in the day, and cooled by the escape of the heat when those vibrations are discontinued in the night, or intercepted and reflected by clouds?
Is it not thus that fire is amassed, and makes the greatest part of the substance of combustible bodies?
Perhaps, when this globe was first formed, and its original particles took their place at certain distances from the centre, in proportion to their greater or less gravity, the fluid fire, attracted towards that centre, might in great part be obliged, as lightest, to take place above the rest, and thus form the sphere of fire above supposed, which would afterward be continually diminishing by the substance it afforded to organized bodies, and the quantity restored to it again by the burning or other separating of the parts of those bodies?
Is not the natural heat of animals thus produced, by separating in digestion the parts of food, and setting their fire at liberty?
Is it not this sphere of fire which kindles the wandering globes that sometimes pass through it in our course round the sun, have their surface kindled by it, and burst when their included air is greatly rarefied by the heat on their burning surfaces?
May it not have been from such considerations that the ancient philosophers supposed a sphere of fire to exist above the air of our atmosphere?
B. Franklin.
Of Lightning; and the Methods now used in America for the securing Buildings and Persons from its mischievous Effects.
Experiments made in electricity first gave philosophers a suspicion that the matter of lightning was the same with the electric matter. Experiments afterward made on lightning obtained from the clouds by pointed rods, received into bottles, and subjected to every trial, have since proved this suspicion to be perfectly well founded; and that, whatever properties we find in electricity, are also the properties of lightning.
This matter of lightning or of electricity is an extreme subtile fluid, penetrating other bodies, and subsisting in them, equally diffused.
When, by any operation of art or nature, there happens to be a greater proportion of this fluid in one body than in another, the body which has most will communicate to that which has least, till the proportion becomes equal; provided the distance between them be not too great; or, if it is too great, till there be proper conductors to convey it from one to the other.
If the communication be through the air without any conductor, a bright light is seen between the bodies, and a sound is heard. In our small experiments we call this light and sound the electric spark and snap; but in the great operations of nature the light is what we calllightning, and the sound (produced at the same time, though generally arriving later at our ears than the light does to our eyes) is, with its echoes, calledthunder.
If the communication of this fluid is by a conductor, it may be without either light or sound, the subtile fluid passing in the substance of the conductor.
If the conductor be good and of sufficient bigness, the fluid passes through it without hurting it. If otherwise, it is damaged or destroyed.
All metals and water are good conductors. Other bodies may become conductors by having some quantity of water in them, as wood and other materials used in building; but, not having much water in them, they are not good conductors, and, therefore, are often damaged in the operation.
Glass, wax, silk, wool, hair, feathers, and even wood, perfectly dry, are non-conductors: that is, they resist instead of facilitating the passage of this subtile fluid.
When this fluid has an opportunity of passing through two conductors, one good and sufficient, as of metal, the other not so good, it passes in the best, and will follow it in any direction.
The distance at which a body charged with this fluid will discharge itself suddenly, striking through the air into another body that is not charged or not so highly charged, is different according to the quantity of the fluid, the dimensions and form of the bodies themselves, and the state of the air between them. This distance, whatever it happens to be, between any two bodies, is called thestriking distance, as, till they come within that distance of each other, no stroke will be made.
The clouds have often more of this fluid, in proportion, than the earth; in which case, as soon as they come near enough (that is, within the striking distance) or meet with a conductor, the fluid quits them and strikes into the earth. A cloud fully charged with this fluid, if so high as to be beyond the striking distance from the earth, passes quietly without making noise or giving light, unless it meets with other clouds that have less.
Tall trees and lofty buildings, as the towers and spires of churches, become sometimes conductors between the clouds and the earth; but, not being good ones, that is, not conveying the fluid freely, they are often damaged.
Buildings that have their roofs covered with leador other metal, the spouts of metal continued from the roof into the ground to carry off the water, are never hurt by lightning as, whenever it falls on such a building, it passes in the metals and not in the walls.
When other buildings happen to be within the striking distance from such clouds, the fluid passes in the walls, whether of wood, brick, or stone, quitting the walls only when it can find better conductors near them, as metal rods, bolts, and hinges of windows or doors, gilding on wainscot or frames of pictures, the silvering on the backs of looking-glasses, the wires for bells, and the bodies of animals, as containing watery fluids. And, in passing through the house, it follows the direction of these conductors, taking as many in its way as can assist it in its passage, whether in a straight or crooked line, leaping from one to the other, if not far distant from each other, only rending the wall in the spaces where these partial good conductors are too distant from each other.
An iron rod being placed on the outside of a building, from the highest part continued down into the moist earth in any direction, straight or crooked, following the form of the roof or parts of the building, will receive the lightning at the upper end, attracting it so as to prevent its striking any other part, and affording it a good conveyance into the earth, will prevent its damaging any part of the building.
A small quantity of metal is found able to conduct a great quantity of this fluid. A wire no bigger than a goosequill has been known to conduct (with safety to the building as far as the wire was continued) a quantity of lightning that did prodigious damage both above and below it; and probably larger rods are not necessary, though it is common in America to make them of half an inch, some of three quarters or an inch diameter.
The rod may be fastened to the wall, chimney&c., with staples of iron. The lightning will not leave the rod (a good conductor) through those staples. It would rather, if any were in the walls, pass out of it into the rod, to get more readily by that conductor into the earth.
If the building be very large and extensive, two or more rods may be placed at different parts, for greater security.
Small ragged parts of clouds, suspended in the air between the great body of clouds and the earth (like leaf gold in electrical experiments) often serve as partial conductors for the lightning, which proceeds from one of them to another, and by their help comes within the striking distance to the earth or a building. It therefore strikes through those conductors a building that would otherwise be out of the striking distance.
Long sharp points communicating with the earth, and presented to such parts of clouds, drawing silently from them the fluid they are charged with, they are then attracted to the cloud, and may leave the distance so great as to be beyond the reach of striking.
It is therefore that we elevate the upper end of the rod six or eight feet above the highest part of the building, tapering it gradually to a fine sharp point, which is gilt to prevent its rusting.
Thus the pointed rod either prevents the stroke from the cloud, or, if a stroke is made, conducts it to the earth with safety to the building.
The lower end of the rod should enter the earth so deep as to come at the moist part, perhaps two or three feet; and if bent when under the surface so as to go in a horizontal line six or eight feet from the wall, and then bent again downward three or four feet, it will prevent damage to any of the stones of the foundation.
A person apprehensive of danger from lightning, happening during the time of thunder to be in ahouse not so secured, will do well to avoid sitting near the chimney, near a looking-glass, or any gilt pictures or wainscot; the safest place is the middle of the room (so it be not under a metal lustre suspended by a chain), sitting on one chair and laying the feet up in another. It is still safer to bring two or three mattresses or beds into the middle of the room, and, folding them up double, place the chair upon them; for they not being so good conductors as the walls, the lightning will not choose an interrupted course through the air of the room and the bedding, when it can go through a continued better conductor, the wall. But where it can be had, a hammock or swinging bed, suspended by silk cords equally distant from the walls on every side, and from the ceiling and floor above and below, affords the safest situation a person can have in any room whatever; and what, indeed, may be deemed quite free from danger of any stroke by lightning.
B. Franklin.
Paris, September, 1767.
To Peter Collinson, London.
ELECTRICAL KITE.
Philadelphia, October 16, 1752.
As frequent mention is made in public papers from Europe of the success of the Philadelphia experiment for drawing the electric fire from clouds by means of pointed rods of iron erected on high buildings, &c., it may be agreeable to the curious to be informed that the same experiment has succeeded in Philadelphia, though made in a different and more easy manner, which is as follows:
Make a small cross of two light strips of cedar, the arms so long as to reach to the four corners of a large thin silk handkerchief when extended; tie the corners of the handkerchief to the extremitiesof the cross, so you have the body of a kite, which, being properly accommodated with a tail, loop, and string, will rise in the air like those made of paper; but this, being of silk, is fitter to bear the wet and wind of a thunder-gust without tearing. To the top of the upright stick of the cross is to be fixed a very sharp-pointed wire, rising a foot or more above the wood. To the end of the twine next the hand is to be tied a silk riband, and where the silk and twine join, a key may be fastened. This kite is to be raised when a thunder-gust appears to be coming on, and the person who holds the string must stand within a door or window, or under some cover, so that the silk riband may not be wet; and care must be taken that the twine does not touch the frame of the door or window. As soon as any of the thunder-clouds come over the kite, the pointed wire will draw the electric fire from them, and the kite, with all the twine, will be electrified, and the loose filaments of the twine will stand out every way, and be attracted by an approaching finger. And when the rain has wetted the kite and twine, so that it can conduct the electric fire freely, you will find it stream out plentifully from the key on the approach of your knuckle. At this key the vial may be charged; and from electric fire thus obtained, spirits may be kindled, and all the other electric experiments be performed, which are usually done by the help of a rubbed glass globe or tube, and thereby the sameness of the electric matter with that of lightning completely demonstrated.
B. Franklin.
Physical and Meteorological Observations, Conjectures, and Suppositions.—Read at the Royal Society, June 3, 1756.
The particles of air are kept at a distance from each other by their mutual repulsion * * *
Whatever particles of other matter (not endued with that repellancy) are supported in air, must adhere to the particles of air, and be supported by them; for in the vacancies there is nothing they can rest on.
Air and water mutually attract each other. Hence water will dissolve in air, as salt in water.
The specific gravity of matter is not altered by dividing the matter, though the superfices be increased. Sixteen leaden bullets, of an ounce each, weigh as much in water as one of a pound, whose superfices is less.
Therefore the supporting of salt in water is not owing to its superfices being increased.
A lump of salt, though laid at rest at the bottom of a vessel of water, will dissolve therein, and its parts move every way, till equally diffused in the water; therefore there is a mutual attraction between water and salt. Every particle of water assumes as many of salt as can adhere to it; when more is added, it precipitates, and will not remain suspended.
Water, in the same manner, will dissolve in air, every particle of air assuming one or more particles of water. When too much is added, it precipitates in rain.
But there not being the same contiguity between the particles of air as of water, the solution of water in air is not carried on without a motion of the air so as to cause a fresh accession of dry particles.
Part of a fluid, having more of what it dissolves, will communicate to other parts that have less. Thus very salt water, coming in contact with fresh, communicates its saltness till all is equal, and the sooner if there is a little motion of the water. * * *
Air, suffering continual changes in the degrees of its heat, from various causes and circumstances, and, consequently, changes in its specific gravity, must therefore be in continual motion.
A small quantity of fire mixed with water (or degree of heat therein) so weakens the cohesion of its particles, that those on the surface easily quit it and adhere to the particles of air.
Air moderately heated will support a greater quantity of water invisibly than cold air; for its particles being by heat repelled to a greater distance from each other, thereby more easily keep the particles of water that are annexed to them from running into cohesions that would obstruct, refract, or reflect the light.
Hence, when we breathe in warm air, though the same quantity of moisture may be taken up from the lungs as when we breathe in cold air, yet that moisture is not so visible.
Water being extremely heated,i. e., to the degree of boiling, its particles, in quitting it, so repel each other as to take up vastly more space than before and by that repellancy support themselves, expelling the air from the space they occupy. That degree of heat being lessened, they again mutually attract, and having no air particles mixed to adhere to, by which they might be supported and kept at a distance, they instantly fall, coalesce, and become water again.
The water commonly diffused in our atmosphere never receives such a degree of heat from the sun or other cause as water has when boiling; it is not, therefore, supported by such heat, but by adhering to air. * * *
A particle of air loaded with adhering water or any other matter, is heavier than before, and would descend.
The atmosphere supposed at rest, a loaded descending particle must act with a force on the particles it passes between or meets with sufficient to overcome, in some degree, their mutual repellancy, and push them nearer to each other. * * *
Every particle of air, therefore, will bear any load inferior to the force of these repulsions.
Hence the support of fogs, mists, clouds.
Very warm air, clear, though supporting a very great quantity of moisture, will grow turbid and cloudy on the mixture of colder air, as foggy, turbid air will grow clear by warming.
Thus the sun, shining on a morning fog, dissipates it; clouds are seen to waste in a sunshiny day.
But cold condenses and renders visible the vapour: a tankard or decanter filled with cold water will condense the moisture of warm, clear air on its outside, where it becomes visible as dew, coalesces into drops, descends in little streams.
The sun heats the air of our atmosphere most near the surface of the earth; for there, besides the direct rays, there are many reflections. Moreover, the earth itself, being heated, communicates of its heat to the neighbouring air.
The higher regions, having only the direct rays of the sun passing through them, are comparatively very cold. Hence the cold air on the tops of mountains, and snow on some of them all the year, even in the torrid zone. Hence hail in summer.
If the atmosphere were, all of it (both above and below), always of the same temper as to cold or heat, then the upper air would always berarerthan the lower, because the pressure on it is less; consequently lighter, and, therefore, would keep its place.
But the upper air may be more condensed by cold than the lower air by pressure; the lower more expanded by heat than the upper for want of pressure. In such case the upper air will become the heavier, the lower the lighter.
The lower region of air being heated and expanded, heaves up and supports for some time the colder, heavier air above, and will continue to support it while the equilibrium is kept. Thus water issupported in an inverted open glass, while the equilibrium is maintained by the equal pressure upward of the air below; but the equilibrium by any means breaking, the water descends on the heavier side, and the air rises into its place.
The lifted heavy cold air over a heated country becoming by any means unequally supported or unequal in its weight, the heaviest part descends first, and the rest follows impetuously. Hence gusts after heats, and hurricanes in hot climates. Hence the air of gusts and hurricanes is cold, though in hot climates and seasons; it coming from above.
The cold air descending from above, as it penetrates our warm region full of watery particles, condenses them, renders them visible, forms a cloud thick and dark, overcasting sometimes, at once, large and extensive; sometimes, when seen at a distance, small at first, gradually increasing; the cold edge or surface of the cloud condensing the vapours next it, which form smaller clouds that join it, increase its bulk, it descends with the wind and its acquired weight, draws nearer the earth, grows denser with continual additions of water, and discharges heavy showers.
Small black clouds thus appearing in a clear sky, in hot climates portend storms, and warn seamen to hand their sails.
The earth turning on its axis in about twenty-four hours, the equatorial parts must move about fifteen miles in each minute; in northern and southern latitudes this motion is gradually less to the poles, and there nothing.
If there was a general calm over the face of the globe, it must be by the air's moving in every part as fast as the earth or sea it covers. * * *
The air under the equator and between the tropics being constantly heated and rarefied by the sun, rises. Its place is supplied by air from northernand southern latitudes, which, coming from parts wherein the earth and air had less motion, and not suddenly acquiring the quicker motion of the equatorial earth, appears an east wind blowing westward; the earth moving from west to east, and slipping under the air.[37]
Thus, when we ride in a calm, it seems a wind against us: if we ride with the wind, and faster, even that will seem a small wind against us.
The air rarefied between the tropics, and rising, must flow in the higher region north and south. Before it rose it had acquired the greatest motion the earth's rotation could give it. It retains some degree of this motion, and descending in higher latitudes, where the earth's motion is less, will appear a westerly wind, yet tending towards the equatorial parts, to supply the vacancy occasioned by the air of the lower regions flowing thitherward.
Hence our general cold winds are about northwest, our summer cold gusts the same.
The air in sultry weather, though not cloudy, has a kind of haziness in it, which makes objects at a distance appear dull and indistinct. This haziness is occasioned by the great quantity of moisture equally diffused in that air. When, by the cold wind blowing down among it, it is condensed into clouds, and falls in rain, the air becomes purer and clearer. Hence, after gusts, distant objects appear distinct, their figures sharply terminated.
Extreme cold winds congeal the surface of the earth by carrying off its fire. Warm winds afterward blowing over that frozen surface will be chilled by it. Could that frozen surface be turned under, and warmer turned up from beneath it, those warm winds would not be chilled so much.
The surface of the earth is also sometimes muchheated by the sun: and such heated surface, not being changed, heats the air that moves over it.
Seas, lakes, and great bodies of water, agitated by the winds, continually change surfaces; the cold surface in winter is turned under by the rolling of the waves, and a warmer turned up; in summer the warm is turned under, and colder turned up. Hence the more equal temper of seawater, and the air over it. Hence, in winter, winds from the sea seem warm, winds from the land cold. In summer the contrary.
Therefore the lakes northwest of us,[38]as they are not so much frozen, nor so apt to freeze as the earth, rather moderate than increase the coldness of our winter winds.
The air over the sea being warmer, and, therefore, lighter in winter than the air over the frozen land, may be another cause of our general northwest winds, which blow off to sea at right angles from our North American coast. The warm, light sea-air rising, the heavy, cold land-air pressing into its place.
Heavy fluids, descending, frequently form eddies or whirlpools, as is seen in a funnel, where the water acquires a circular motion, receding every way from a centre, and leaving a vacancy in the middle, greatest above, and lessening downward, like a speaking-trumpet, its big end upward.
Air, descending or ascending, may form the same kind of eddies or 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 someplaces, 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 avacuum, 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.
[37]See a paper on this subject, by the late ingenious Mr. Hadley, in the Philadelphia Transactions, wherein this hypothesis of explaining the tradewinds first appeared.
[37]See a paper on this subject, by the late ingenious Mr. Hadley, in the Philadelphia Transactions, wherein this hypothesis of explaining the tradewinds first appeared.
[38]In Pennsylvania.
[38]In Pennsylvania.
To Dr. Perkins.
Water-spouts and Whirlwinds compared.—Read at the Royal Society, June 24, 1753.
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 theTransactions, and on other relations I have met with. Perhaps, while I am writing, somenew 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 theTransactions, 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 favourmy 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 itmy 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 theTransactions, 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 theTransactions, 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, meetwith 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 houseinward, or forceinthe 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 aminformed, 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 willbegin 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, theyascendby a spiral motion, in the same manner as the waterdescendsspirally 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.
Betweena a a aandb b b bI 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 betweena a a aandb b b bthat 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 betweena a a aandb b b bwill 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 beas 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 endit 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 thana 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 themarvellous, 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 betweena a a aandb 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 arrowsaandb, and that between the arrowscandd, will appear much darker than that betweenbandc, 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.