or mixing the various kinds of it, or changing its form and appearance
by different compositions of it; but does not extend to the making or
creating new matter, or annihilating the old. Thus, if fire be an
original element or kind of matter, its quantity is fixed and permanent
in the universe. We cannot destroy any part of it, or make addition to
it; we can only separate it from that which confines it, and so set it
at liberty; as when we put wood in a situation to be burned, or
transfer it from one solid to another, as when we make lime by burning
stone, a part of the fire dislodged in the fuel being left in the stone.
May not this fluid, when at liberty, be capable of penetrating and
entering into all bodies, organized or not, quitting easily in totality
those not organized, and quitting easily in part those which are; the
part assumed and fixed remaining till the body is dissolved?
Is it not this fluid which keeps asunder the particles of air,
permitting them to approach, or separating them more in proportion as
its quantity is diminished or augmented?
Is it not the greater gravity of the particles of air which forces the
particles of this fluid to mount with the matters to which it is
attached, as smoke or vapour?
Does it not seem to have a greater affinity with water, since it will
quit a solid to unite with that fluid, and go off with it in vapour,
leaving the solid cold to the touch, and the degree measurable by the
thermometer?
The vapour rises attached to this fluid, but at a certain height they
separate, and the vapour descends in rain, retaining but little of it,
in snow or hail less. What becomes of that fluid? Does it rise above our
atmosphere, and mix with the universal mass of the same kind?
Or does a spherical stratum of it, denser, as less mixed with air,
attracted by this globe, and repelled or pushed up only to a certain
height from its surface by the greater weight of air, remain there
surrounding the globe, and proceeding with it round the sun?
In such case, as there may be a continuity of communication of this
fluid through the air quite down to the earth, is it not by the
vibrations given to it by the sun that light appears to us? And may it
not be that every one of the infinitely small vibrations, striking
common matter with a certain force, enters its substance, is held there
by attraction, and augmented by succeeding vibrations till the matter
has received as much as their force can drive into it?
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 call
_lightning_, 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, called _thunder_.
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 the _striking 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 lead or 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 a house 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 extremities of 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 be
_rarer_ than 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 is supported 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
northern and 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]
[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.
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 much heated 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.
[38] In Pennsylvania.
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.