uneasy sensation of cold than dry ones; because (to speak like an
electrician) they _conduct_ better; that is, are better fitted to
convey away the heat from our bodies. The body cannot feel _without_
itself; our sensation of cold is not in the air _without_ the body,
but in those parts of the body which have been deprived of their
heat by the air. My desk, and its lock, are, I suppose, of the same
temperament when they have been long exposed to the same air; but
now if I lay my hand on the wood, it does not seem so cold to me as
the lock; because (as I imagine) wood is not so good a conductor,
to receive and convey away the heat from my skin, and the adjacent
flesh, as metal is. Take a piece of wood, of the size and shape of
a dollar, between the thumb and finger of one hand, and a dollar,
in like manner, with the other hand; place the edges of both, at
the same time, in the flame of a candle; and though the edge of the
wooden piece takes flame, and the metal piece does not, yet you will
be obliged to drop the latter before the former, it conducting the
heat more suddenly to your fingers. Thus we can, without pain, handle
glass and china cups filled with hot liquors, as tea, &c. but not
silver ones. A silver tea-pot must have a wooden handle. Perhaps it
is for the same reason that woollen garments keep the body warmer
than linen ones equally thick; woollen keeping the natural heat in,
or, in other words, not conducting it out to air.
In regard to water-spouts, having, in a long letter to a gentleman
of the same sentiment with you as to their direction, said all
that I have to say in support of my opinion; I need not repeat the
arguments therein contained, as I intend to send you a copy of it by
some other opportunity, for your perusal. I imagine you will find all
the appearances you saw, accounted for by my hypothesis. I thank you
for communicating the account of them. At present I would only say,
that the opinion of winds being generated in clouds by fermentation,
is new to me, and I am unacquainted with the facts on which it is
founded. I likewise find it difficult to conceive of winds confined
in the body of clouds, which I imagine have little more solidity than
the fogs on the earth's surface. The objection from the freshness
of rain-water is a strong one, but I think I have answered it in the
letter above-mentioned, to which I must beg leave, at present, to
refer you.
[In Mr. Collinson's edition, there followed here, several extracts,
on water-spouts, from Dampier's Voyages, which, as Dampier's book is
by no means scarce, and is consequently accessible to the reader, we
have omitted, and shall content ourselves with giving the references.
The extracts are three. The first is from Vol. I. p. 451. The second
and third from Vol. III. p. 182 and 223.]
_Gentleman of New York in Reply._
Read at the Royal Society, December 6, 1756.
_April 2, 1754._
Any knowledge I have of the winds, and other changes which happen in
the atmosphere, is so very defective, that it does not deserve the
name; neither have I received any satisfaction from the attempts of
others on this subject. It deserves then your thoughts, as a subject
in which you may distinguish yourself, and be useful.
Your notion of some things conducting heat or cold better than others
pleases me, and I wish you may pursue the scent. If I remember right,
Dr. Boerhaave, in his chymistry, thinks that heat is propagated
by the vibration of a subtle elastic fluid, dispersed through the
atmosphere and through all bodies. Sir Isaac Newton says, there
are many phenomena to prove the existence of such a fluid; and
this opinion has my assent to it. I shall only observe that it is
essentially different from that which I call ether; for ether,
properly speaking, is neither a fluid nor elastic; its power consists
in re-acting any action communicated to it, with the same force it
receives the action.
I long to see your explication of water-spouts, but I must tell you
before-hand, that it will not be easy for you to convince me that the
principal phenomena were not occasioned by a stream of wind issuing
with great force, my eyes and ears both concurring to give me this
sentiment, I could have no more evidence than to feel the effects,
which I had no inclination to do.
It surprises me a little, that wind, generated by fermentation, is
new to you, since it may be every day observed in fermenting liquor.
You know with what force fermenting liquors will burst the vessels
which contain them, if the generated wind have not vent; and with
what force it issues on giving it a small vent, or by drawing the
cork of a bottle. Dr. Boerhaave says, that the steam issuing from
fermenting liquors received through a very small vent-hole, into the
nose, will kill as suddenly and certainly as lightning. That air is
generated by fermentation, I think you will find fully proved in Dr.
Hales's Analysis of the Air, in his Vegetable Statics. If you have
not read the book, you have a new pleasure to come.
The solution you give to the objection I made from the contrary winds
blowing from the opposite sides of the mountains, from their being
eddies, does not please me, because the extent of these winds is by
far too large to be occasioned by any eddy. It is forty miles from
New York to our mountains, through which Hudson's River passes. The
river runs twelve miles in the mountains, and from the north side of
the mountains it is about ninety miles to Albany. I have myself been
on board a vessel more than once, when we have had a strong northerly
wind against us, all the way from New York, for two or three days.
We have met vessels from Albany, who assured us, that, on the other
side of the mountains, they had, at the same time, a strong continued
southerly wind against them; and this frequently happens.
I have frequently seen, both on the river, in places where there
could be no eddy-weeds, and on the open sea, two vessels sailing with
contrary winds, within half a mile of each other; but this happens
only in easy winds, and generally calm in other places near these
winds.
You have, no doubt, frequently observed a single cloud pass, from
which a violent gust of wind issues, but of no great extent. I have
observed such a gust make a lane through the woods, of some miles
in length, by laying the trees flat to the ground, and not above
eight or ten chains in breadth. Though the violence of the wind be
in the same direction in which the cloud moves and precedes it, yet
wind issues from all sides of it; so that supposing the cloud moves
south-easterly, those on the north-east side of it feel a south-west
wind, and others on the south-west side, a north-east. And where the
cloud passes over, we frequently have a south-east wind from the
hinder part of it, but none violent, except the wind in the direction
in which the cloud moves. To shew what it is which prevents the wind
from issuing out equally on all sides, is not an easy problem to me,
and I shall not attempt to solve it; but when you shall show what
it is which restrains the electrical fluid from spreading itself
into the air surrounding it, when it rushes with great violence
through the air along, or in the conductor, for a great extent in
length, then I may hope to explain the other problem, and remove the
difficulty we have in conceiving it.
TO PETER COLLINSON, ESQ. LONDON.
_Account of a Whirlwind at Maryland._
_Philadelphia, Aug. 25, 1755._
DEAR SIR,
As you have my former papers on whirlwinds, &c. I now send you an
account of one which I had lately an opportunity of seeing and
examining myself.
Being in Maryland, riding with Colonel Tasker, and some other
gentlemen, to his country seat, where I and my son were entertained
by that amiable and worthy man with great hospitality and kindness,
we saw, in the vale below us, a small whirlwind beginning in the
road, and shewing itself by the dust it raised and contained. It
appeared in the form of a sugar-loaf, spinning on its point, moving
up the hill towards us, and enlarging as it came forward. When it
passed by us, its smaller part near the ground appeared no bigger
than a common barrel, but widening upwards, it seemed, at forty or
fifty feet high, to be twenty or thirty feet in diameter. The rest of
the company stood looking after it, but my curiosity being stronger,
I followed it, riding close by its side, and observed its licking
up, in its progress, all the dust that was under its smaller part.
As it is a common opinion that a shot, fired through a water-spout,
will break it, I tried to break this little whirlwind, by striking my
whip frequently through it, but without any effect. Soon after, it
quitted the road and took into the woods, growing every moment larger
and stronger, raising, instead of dust, the old dry leaves with which
the ground was thick covered, and making a great noise with them
and the branches of the trees, bending some tall trees round in a
circle swiftly and very surprisingly, though the progressive motion
of the whirl was not so swift but that a man on foot might have kept
pace with it, but the circular motion was amazingly rapid. By the
leaves it was now filled with, I could plainly perceive that the
current of air they were driven by moved upwards in a spiral line;
and when I saw the passing whirl continue entire, after leaving the
trunks and bodies of large trees which it had enveloped, I no longer
wondered that my whip had no effect on it in its smaller state. I
accompanied it about three quarters of a mile, till some limbs of
dead trees, broken off by the whirl, flying about, and falling near
me, made me more apprehensive of danger; and then I stopped, looking
at the top of it as it went on, which was visible, by means of the
leaves contained in it, for a very great height above the trees.
Many of the leaves, as they got loose from the upper and widest
part, were scattered in the wind; but so great was their height in
the air, that they appeared no bigger than flies. My son, who was,
by this time, come up with me, followed the whirlwind till it left
the woods, and crossed an old tobacco-field, where, finding neither
dust nor leaves to take up, it gradually became invisible below as
it went away over that field. The course of the general wind then
blowing was along with us as we travelled, and the progressive motion
of the whirlwind was in a direction nearly opposite, though it did
not keep a strait line, nor was its progressive motion uniform, it
making little sallies on either hand as it went, proceeding sometimes
faster, and sometimes slower, and seeming sometimes for a few seconds
almost stationary, then starting forwards pretty fast again. When
we rejoined the company, they were admiring the vast height of the
leaves now brought by the common wind, over our heads. These leaves
accompanied us as we travelled, some falling now and then round about
us, and some not reaching the ground till we had gone near three
miles from the place where we first saw the whirlwind begin. Upon my
asking Colonel Tasker if such whirlwinds were common in Maryland,
he answered pleasantly, No, not at all common, but we got this on
purpose to treat Mr. Franklin. And a very high treat it was to,
Dear Sir,
Your affectionate friend and humble servant,
B. FRANKLIN.
TO MR. ALEXANDER SMALL, LONDON.
_On the North-East Storms in North America._
_May 12, 1760._
DEAR SIR,
Agreeable to your request, I send you my reasons for thinking that
our north-east storms in North America begin first, in point of
time, in the south-west parts: that is to say, the air in Georgia,
the farthest of our colonies to the south-west, begins to move
south-westerly before the air of Carolina, which is the next colony
north-eastward; the air of Carolina has the same motion before the
air of Virginia, which lies still more north-eastward; and so on
north-easterly through Pensylvania, New-York, New-England, &c. quite
to Newfoundland.
These north-east storms are generally very violent, continue
sometimes two or three days, and often do considerable damage in the
harbours along the coast. They are attended with thick clouds and
rain.
What first gave me this idea, was the following circumstance. About
twenty years ago, a few more or less, I cannot from my memory be
certain, we were to have an eclipse of the moon at Philadelphia, on
a Friday evening, about nine o'clock. I intended to observe it, but
was prevented by a north-east storm, which came on about seven, with
thick clouds as usual, that quite obscured the whole hemisphere. Yet
when the post brought us the Boston news-paper, giving an account of
the effects of the same storm in those parts, I found the beginning
of the eclipse had been well observed there, though Boston lies N. E.
of Philadelphia about four hundred miles. This puzzled me, because
the storm began with us so soon as to prevent any observation, and
being a north-east storm, I imagined it must have begun rather sooner
in places farther to the north-east-ward than it did at Philadelphia.
I therefore mentioned it in a letter to my brother, who lived at
Boston; and he informed me the storm did not begin with them till
near eleven o'clock, so that they had a good observation of the
eclipse: and upon comparing all the other accounts I received from
the several colonies, of the time of beginning of the same storm, and
since that of other storms of the same kind, I found the beginning to
be always later the farther north-eastward. I have not my notes with
me here in England, and cannot, from memory, say the proportion of
time to distance, but I think it is about an hour to every hundred
miles.
From thence I formed an idea of the cause of these storms, which I
would explain by a familiar instance or two.--Suppose a long canal
of water stopped at the end by a gate. The water is quite at rest
till the gate is open, then it begins to move out through the gate;
the water next the gate is first in motion, and moves towards the
gate; the water next to that first water moves next, and so on
successively, till the water at the head of the canal is in motion,
which is last of all. In this case all the water moves indeed towards
the gate, but the successive times of beginning motion are the
contrary way, viz. from the gate backwards to the head of the canal.
Again, suppose the air in a chamber at rest, no current through the
room till you make a fire in the chimney. Immediately the air in the
chimney being rarefied by the fire rises; the air next the chimney
flows in to supply its place, moving towards the chimney; and, in
consequence, the rest of the air successively, quite back to the
door. Thus to produce our north-east storms, I suppose some great
heat and rarefaction of the air in or about the Gulph of Mexico; the
air thence rising has its place supplied by the next more northern,
cooler, and therefore denser and heavier, air; that, being in motion,
is followed by the next more northern air, &c. &c. in a successive
current, to which current our coast and inland ridge of mountains
give the direction of north-east, as they lie N. E. and S. W.
This I offer only as an hypothesis to account for this particular
fact; and perhaps, on farther examination, a better and truer may be
found. I do not suppose all storms generated in the same manner. Our
north-west thunder-gusts in America, I know are not; but of them I
have written my opinion fully in a paper which you have seen.
I am, &c.
B. FRANKLIN.
_Meteorological Imaginations and Conjectures_[11].
There seems to be a region higher in the air over all countries,
where it is always winter, where frost exists continually, since in
the midst of summer, on the surface of the earth, ice falls often
from above in the form of hail.
Hailstones, of the great weight we sometimes find them, did not
probably acquire their magnitude before they began to descend. The
air, being eight hundred times rarer than water, is unable to support
it but in the shape of vapour, a state in which its particles are
separated. As soon as they are condensed by the cold of the upper
region, so as to form a drop, that drop begins to fall. If it freezes
into a grain of ice, that ice descends. In descending, both the
drop of water and the grain of ice are augmented by particles of
the vapour they pass through in falling, and which they condense by
coldness, and attach to themselves.
It is possible that, in summer, much of what is rain, when it arrives
at the surface of the earth, might have been snow when it began its
descent; but being thawed, in passing through the warm air near the
surface, it is changed from snow into rain.
How immensely cold must be the original particle of hail, which
forms the centre of the future hailstone, since it is capable of
communicating sufficient cold, if I may so speak, to freeze all the
mass of vapour condensed round it, and form a lump of perhaps six or
eight ounces in weight!
When, in summer time, the sun is high, and continues long every day
above the horizon, his rays strike the earth more directly, and with
longer continuance, than in the winter; hence the surface is more
heated, and to a greater depth, by the effect of those rays.
When rain falls on the heated earth, and soaks down into it, it
carries down with it a great part of the heat, which by that means
descends still deeper.
The mass of earth, to the depth perhaps of thirty feet, being thus
heated to a certain degree, continues to retain its heat for some
time. Thus the first snows that fall in the beginning of winter,
seldom lie long on the surface, but are soon melted, and soon
absorbed. After which, the winds, that blow over the country on which
the snows had fallen, are not rendered so cold as they would have
been, by those snows, if they had remained, and thus the approach of
the severity of winter is retarded; and the extreme degree of its
cold is not always at the time we might expect it, viz. when the sun
is at its greatest distance, and the day shortest, but some time
after that period, according to the English proverb, which says, "as
the day lengthens, the cold strengthens;" the causes of refrigeration
continuing to operate, while the sun returns too slowly, and his
force continues too weak to counteract them.
During several of the summer months of the year 1783, when the
effects of the sun's rays to heat the earth in these northern regions
should have been the greatest, there existed a constant fog over all
Europe, and great part of North America. This fog was of a permanent
nature: it was dry, and the rays of the sun seemed to have little
effect towards dissipating it, as they easily do a moist fog, arising
from water. They were indeed rendered so faint in passing through
it, that when collected in the focus of a burning glass, they would
scarce kindle brown paper. Of course, their summer effect in heating
the earth was exceedingly diminished.
Hence the surface was early frozen.
Hence the first snows remained on it unmelted, and received continual
additions.
Hence perhaps the winter of 1783-4, was more severe than any that had
happened for many years.
The cause of this universal fog is not yet ascertained. Whether
it was adventitious to this earth, and merely a smoke proceeding
from the consumption by fire of some of those great burning balls
or globes which we happen to meet with in our rapid course round
the sun, and which are sometimes seen to kindle and be destroyed
in passing our atmosphere, and whose smoke might be attracted and
retained by our earth; or whether it was the vast quantity of smoke,
long continuing to issue during the summer from Hecla, in Iceland,
and that other volcano which arose out of the sea near that island,
which smoke might be spread by various winds, over the northern part
of the world, is yet uncertain.
It seems however worth the enquiry, whether other hard winters,
recorded in history, were preceded by similar permanent and widely
extended summer fogs. Because, if found to be so, men might from such
fogs conjecture the probability of a succeeding hard winter, and of
the damage to be expected by the breaking up of frozen rivers in the
spring; and take such measures as are possible and practicable, to
secure themselves and effects from the mischiefs that attended the
last.
_Passy, May 1784._
FOOTNOTE:
[11] This paper is taken from the Memoirs of the Literary and
Philosophical Society of Manchester, Vol. II. page 373. It was
communicated by Dr. Percival, and read December 22, 1784. _Editor._
_Suppositions and Conjectures towards forming an Hypothesis, for
the Explanation of the Aurora Borealis_[12].
1. Air heated by any means, becomes rarefied, and specifically
lighter than other air in the same situation not heated.
2. Air being made thus lighter rises, and the neighbouring cooler
heavier air takes its place.
3. If in the middle of a room you heat the air by a stove, or pot
of burning coals near the floor, the heated air will rise to the
ceiling, spread over the cooler air till it comes to the cold walls;
there, being condensed and made heavier, it descends to supply the
place of that cool air, which had moved towards the stove or fire, in
order to supply the place of the heated air, which had ascended from
the space around the stove or fire.
4. Thus there will be a continual circulation of air in the room;
which may be rendered visible by making a little smoke, for that
smoke will rise and circulate with the air.
5. A similar operation is performed by nature on the air of this
globe. Our atmosphere is of a certain height, perhaps at a medium [___]
miles: above that height it is so rare as to be almost a vacuum. The
air heated between the tropics is continually rising; its place is
supplied by northerly and southerly winds, which come from the cooler
regions.
6. The light heated air, floating above the cooler and denser, must
spread northward and southward; and descend near the two poles, to
supply the place of the cool air, which had moved towards the equator.
7. Thus a circulation of air is kept up in our atmosphere, as in the
room above-mentioned.
8. That heavier and lighter air may move in currents of different and
even opposite direction, appears sometimes by the clouds that happen
to be in those currents, as plainly as by the smoke in the experiment
above-mentioned. Also in opening a _door_ between two chambers, one
of which has been warmed, by holding a candle near the top, near the
bottom, and near the middle, you will find a strong current of warm
air passing out of the warmed room above, and another of cool air
entering below; while in the middle there is little or no motion.
9. The great quantity of vapour rising between the tropics forms
clouds, which contain much electricity.
Some of them fall in rain, before they come to the polar regions.
10. If the rain be received in an isolated vessel, the vessel will be
electrified; for every drop brings down some electricity with it.
11. The same is done by snow or hail.
12. The electricity so descending, in temperate climates, is received
and imbibed by the earth.
13. If the clouds are not sufficiently discharged by this gradual
operation, they sometimes discharge themselves suddenly by striking
into the earth, where the earth is fit to receive their electricity.
14. The earth in temperate and warm climates is generally fit to
receive it, being a good conductor.
15. A certain quantity of heat will make some bodies good conductors,
that will not otherwise conduct.
16. Thus wax rendered fluid, and glass softened by heat, will both of
them conduct.
17. And water, though naturally a good conductor, will not conduct
well, when frozen into ice by a common degree of cold; not at all,
where the cold is extreme.
18. Snow falling upon frozen ground has been found to retain its
electricity; and to communicate it to an isolated body, when after
falling, it has been driven about by the wind.
19. The humidity, contained in all the equatorial clouds that reach
the polar regions, must there be condensed and fall in snow.
20. The great cake of ice that eternally covers those regions may be
too hard frozen to permit the electricity, descending with that snow,
to enter the earth.
21. It may therefore be _accumulated upon that ice_.
22. The atmosphere being heavier in the polar regions than in the
equatorial, will there be lower; as well from that cause, as from the
smaller effect of the centrifugal force: consequently the distance
of the vacuum above the atmosphere will be less at the poles, than
elsewhere; and probably much less than the distance (upon the surface
of the globe) extending from the pole to those latitudes in which
the earth is so thawed as to receive and imbibe electricity; (the
frost continuing to lat. 80, which is ten degrees, or six hundred
miles from the pole; while the height of the atmosphere there of such
density as to obstruct the motion of the electric fluid, can scarce
be esteemed above [___] miles).
23. The _vacuum_ above is a good conductor.