be explained by this property? During the hot Sunday at Philadelphia,
in June 1750, when the thermometer was up at 100 in the shade, I
sat in my chamber without exercise, only reading or writing, with
no other cloaths on than a shirt, and a pair of long linen drawers,
the windows all open, and a brisk wind blowing through the house,
the sweat ran off the backs of my hands, and my shirt was often
so wet, as to induce me to call for dry ones to put on; in this
situation, one might have expected, that the natural heat of the body
96, added to the heat of the air 100, should jointly have created
or produced a much greater degree of heat in the body; but the fact
was, that my body never grew so hot as the air that surrounded it,
or the inanimate bodies immersed in the same air. For I remember
well, that the desk, when I laid my arm upon it; a chair, when I sat
down in it; and a dry shirt out of the drawer, when I put it on,
all felt exceeding warm to me, as if they had been warmed before a
fire. And I suppose a dead body would have acquired the temperature
of the air, though a living one, by continual sweating, and by the
evaporation of that sweat, was kept cold. May not this be a reason
why our reapers in Pensylvania, working in the open field, in the
clear hot sun-shine common in our harvest-time[15], find themselves
well able to go through that labour, without being much incommoded
by the heat, while they continue to sweat, and while they supply
matter for keeping up that sweat, by drinking frequently of a thin
evaporable liquor, water mixed with rum; but if the sweat stops,
they drop, and sometimes die suddenly, if a sweating is not again
brought on by drinking that liquor, or, as some rather chuse in that
case, a kind of hot punch, made with water, mixed with honey, and a
considerable proportion of vinegar? May there not be in negroes a
quicker evaporation of the perspirable matter from their skins and
lungs, which, by cooling them more, enables them to bear the sun's
heat better than whites do? (if that is a fact, as it is said to be;
for the alledged necessity of having negroes rather than whites, to
work in the West-India fields, is founded upon it) though the colour
of their skins would otherwise make them more sensible of the sun's
heat, since black cloth heats much sooner, and more, in the sun,
than white cloth. I am persuaded, from several instances happening
within my knowledge, that they do not bear cold weather so well as
the whites; they will perish when exposed to a less degree of it,
and are more apt to have their limbs frostbitten; and may not this
be from the same cause? Would not the earth grow much hotter under
the summer-sun, if a constant evaporation from its surface, greater
as the sun shines stronger, did not, by tending to cool it; balance,
in some degree, the warmer effects of the sun's rays? Is it not
owing to the constant evaporation from the surface of every leaf,
that trees, though shone on by the sun, are always, even the leaves
themselves, cool to our sense? at least much cooler than they would
otherwise be? May it not be owing to this, that fanning ourselves
when warm, does really cool us, though the air is itself warm that we
drive with the fan upon our faces; for the atmosphere round, and next
to our bodies, having imbibed as much of the perspired vapour as it
can well contain, receives no more, and the evaporation is therefore
checked and retarded, till we drive away that atmosphere, and bring
drier air in its place, that will receive the vapour, and thereby
facilitate and increase the evaporation? Certain it is, that mere
blowing of air on a dry body does not cool it, as any one may satisfy
himself, by blowing with a bellows on the dry ball of a thermometer;
the mercury will not fall; if it moves at all, it rather rises, as
being warmed by the friction of the air on its surface? To these
queries of imagination, I will only add one practical observation;
that wherever it is thought proper to give ease, in cases of painful
inflammation in the flesh (as from burnings, or the like) by cooling
the part; linen cloths, wet with spirit, and applied to the part
inflamed, will produce the coolness required, better than if wet with
water, and will continue it longer. For water, though cold when first
applied, will soon acquire warmth from the flesh, as it does not
evaporate fast enough; but the cloths wet with spirit, will continue
cold as long as any spirit is left to keep up the evaporation, the
parts warmed escaping as soon as they are warmed, and carrying off
the heat with them.
I am, Sir, &c.
B FRANKLIN.
FOOTNOTE:
[15] Pensylvania is in about lat. 40, and the sun, of course, about
12 degrees higher, and therefore much hotter than in England. Their
harvest is about the end of June, or beginning of July, when the sun
is nearly at the highest.
J. B.[16] ESQ. IN BOSTON, TO B. FRANKLIN.
_Concerning the Light in Sea-Water._
Read at the Royal Society, December 6, 1756.
_November_ 12, 1753.
**** When I was at the eastward, I had an opportunity of observing
the luminous appearance of the sea when disturbed: at the head and
stern of the vessel, when under way, it appeared very bright. The
best opportunity I had to observe it was in a boat, in company with
several gentlemen going from Portsmouth, about three miles, to our
vessel lying at the mouth of Piscataqua River. Soon after we set
off (it being in the evening) we observed a luminous appearance,
where the oars dashed the water. Sometimes it was very bright, and
afterwards, as we rowed along, gradually lessened, till almost
imperceptible, and then re-illumined. This we took notice of several
times in the passage. When I got on board the vessel, I ordered a
pail to be dipped up, full of sea-water, in which, on the water's
being moved, a sparkling light appeared. I took a linen cloth,
and strained some of the water through it, and there was a like
appearance on the cloth, which soon went off; but on rubbing the
cloth with my finger, it was renewed. I then carried the cloth to the
light, but could not perceive any thing upon it which should cause
that appearance.
Several gentlemen were of opinion, that the separated particles
of putrid, animal, and other bodies, floating on the surface of
the sea, might cause that appearance; for putrid fish, &c. they
said, will cause it: and the sea-animals which have died, and
other bodies putrified therein since the creation, might afford
a sufficient quantity of these particles to cover a considerable
portion of the surface of the sea; which particles being differently
dispersed, might account for the different degrees of light in
the appearance above-mentioned. But this account seems liable to
this obvious objection, that as putrid fish, &c. make a luminous
appearance without being moved or disturbed, it might be expected
that the supposed putrid particles on the surface of the sea, should
always appear luminous, where there is not a greater light; and,
consequently, that the whole surface of the sea, covered with those
particles, should always, in dark nights, appear luminous, without
being disturbed. But this is not fact.
Among the rest, I threw out my conjecture, that the said appearance
might be caused by a great number of little animals, floating on the
surface of the sea, which, on being disturbed, might, by expanding
their finns, or otherwise moving themselves, expose such a part
of their bodies as exhibits a luminous appearance, somewhat in
the manner of a glow-worm, or fire-fly: that these animals may be
more numerous in some places than others; and, therefore, that the
appearance above-mentioned being fainter and stronger in different
places, might be owing to that: that certain circumstances of
weather, &c. might invite them to the surface, on which, in a calm,
they might sport themselves and glow; or in storms, being forced up,
make the same appearance.
There is no difficulty in conceiving that the sea may be stocked with
animalcula for this purpose, as we find all nature crowded with
life. But it seems difficult to conceive that such small portions
of matter, even if they were wholly luminous, should affect our
sight; much more so, when it is supposed that only a part of them is
luminous. But, if we consider some other appearances, we may find
the same difficulty to conceive of them; and yet we know they take
place. For instance, the flame of a candle, which, it is said, may
be seen four miles round. The light which fills this circle of eight
miles diameter, was contained, when it first left the candle, within
a circle of half an inch diameter. If the density of light, in these
circumstances, be as those circles to each other, that is, as the
squares of their diameters, the candle-light, when come to the eye,
will be 1027709337600 times rarer than when it quitted the half inch
circle. Now the aperture of the eye, through which the light passes,
does not exceed one-tenth of an inch diameter, and the portion of
the lesser circle, which corresponds to this small portion of the
greater circle, must be proportionably, that is, 1027709337600 times
less than one-tenth of an inch; and yet this infinitely small point
(if you will allow the expression) affords light enough to make it
visible four miles; or, rather, affords light sufficient to affect
the sight at that distance.
The smallness of the animalcula is no objection then to this
conjecture; for supposing them to be ten thousand times less than the
_minimum visibile_, they may, notwithstanding, emit light enough to
affect the eyes, and so to cause the luminous appearance aforesaid.
This conjecture I send you for want of something better ****.
FOOTNOTE:
[16] I. Badoin. _Editor._
TO MR. P. F.[17] IN NEWPORT.
_On the Saltness of Sea-Water._
_London, May 7, 1760._
SIR,
**** It has, indeed, as you observe, been the opinion of some very
great naturalists, that the sea is salt only from the dissolution
of mineral or rock-salt, which its waters happened to meet with.
But this opinion takes it for granted that all water was originally
fresh, of which we can have no proof. I own I am inclined to a
different opinion, and rather think all the water on this globe was
originally salt, and that the fresh water we find in springs and
rivers, is the produce of distillation. The sun raises the vapours
from the sea, which form clouds, and fall in rain upon the land, and
springs and rivers are formed of that rain. As to the rock-salt found
in mines, I conceive, that instead of communicating its saltness to
the sea, it is itself drawn from the sea, and that of course the sea
is now fresher than it was originally. This is only another effect of
nature's distillery, and might be performed various ways.
It is evident from the quantities of sea-shells, and the bones and
teeth of fishes found in high lands, that the sea has formerly
covered them. Then, either the sea has been higher than it now is,
and has fallen away from those high lands, or they have been lower
than they are, and were lifted up out of the water to their present
height, by some internal mighty force, such as we still feel some
remains of, when whole continents are moved by earthquakes. In
either case it may be supposed that large hollows or valleys among
hills, might be left filled with sea-water, which evaporating, and
the fluid part drying away in a course of years, would leave the salt
covering the bottom; and that salt coming afterwards to be covered
with earth, from the neighbouring hills, could only be found by
digging through that earth. Or, as we know from their effects, that
there are deep fiery caverns under the earth, and even under the
sea, if at any time the sea leaks into any of them, the fluid parts
of the water must evaporate from that heat, and pass off through
some volcano, while the salt remains, and by degrees, and continual
acretion, becomes a great mass. Thus the cavern may at length be
filled, and the volcano connected with it cease burning, as many
it is said have done; and future miners, penetrating such cavern,
find what we call a salt-mine. This is a fancy I had on visiting
the salt-mines at Northwich, with my son. I send you a piece of the
rock-salt which he brought up with him out of the mine. ****
I am, Sir, &c.
B. FRANKLIN.
FOOTNOTE:
[17] Peter Franklin. _Editor._
TO MISS STEPHENSON.
_On the Effect of Air on the Barometer, and the Benefits derived
from the Study of Insects._
_Craven Street, June 11, 1760._
'Tis a very sensible question you ask, how the air can affect the
barometer, when its opening appears covered with wood? If indeed it
was so closely covered as to admit of no communication of the outward
air to the surface of the mercury, the change of weight in the air
could not possibly affect it. But the least crevice is sufficient
for the purpose; a pinhole will do the business. And if you could
look behind the frame to which your barometer is fixed, you would
certainly find some small opening.
There are indeed some barometers in which the body of mercury at the
lower end is contained in a close leather bag, and so the air cannot
come into immediate contact with the mercury; yet the same effect is
produced. For the leather being flexible, when the bag is pressed by
any additional weight of air it contracts, and the mercury is forced
up into the tube; when the air becomes lighter, and its pressure
less, the weight of the mercury prevails, and it descends again into
the bag.
Your observation on what you have lately read concerning insects is
very just and solid. Superficial minds are apt to despise those who
make that part of the creation their study, as mere triflers; but
certainly the world has been much obliged to them. Under the care
and management of man, the labours of the little silkworm afford
employment and subsistence to thousands of families, and become an
immense article of commerce. The bee, too, yields us its delicious
honey, and its wax useful to a multitude of purposes. Another insect,
it is said, produces the cochineal, from whence we have our rich
scarlet dye. The usefulness of the cantharides or Spanish flies, in
medicine, is known to all, and thousands owe their lives to that
knowledge. By human industry and observation, other properties of
other insects may possibly be hereafter discovered, and of equal
utility. A thorough acquaintance with the nature of these little
creatures may also enable mankind to prevent the increase of such
as are noxious, or secure us against the mischiefs they occasion.
These things doubtless your books make mention of: I can only add
a particular late instance which I had from a Swedish gentleman of
good credit. In the green timber, intended for ship-building at the
king's yards in that country, a kind of worms were found, which every
year became more numerous and more pernicious, so that the ships were
greatly damaged before they came into use. The king sent Linnæus, the
great naturalist, from Stockholm, to enquire into the affair, and see
if the mischief was capable of any remedy. He found, on examination,
that the worm was produced from a small egg, deposited in the little
roughnesses on the surface of the wood, by a particular kind of fly
or beetle; from whence the worm, as soon as it was hatched, began
to eat into the substance of the wood, and after some time came out
again a fly of the parent kind, and so the species increased. The
season in which the fly laid its eggs, Linnæus knew to be about a
fortnight (I think) in the month of May, and at no other time in the
year. He therefore advised, that some days before that season, all
the green timber should be thrown into the water, and kept under
water till the season was over. Which being done by the king's order,
the flies missing their usual nests, could not increase; and the
species was either destroyed or went elsewhere; and the wood was
effectually preserved, for after the first year, it became too dry
and hard for their purpose.
There is, however, a prudent moderation to be used in studies of
this kind. The knowledge of nature may be ornamental, and it may
be useful, but if to attain an eminence in that, we neglect the
knowledge and practice of essential duties, we deserve reprehension.
For there is no rank in natural knowledge of equal dignity and
importance with that of being a good parent, a good child, a good
husband, or wife, a good neighbour or friend, a good subject or
citizen, that is, in short, a good christian. Nicholas Gimcrack,
therefore, who neglected the care of his family, to pursue
butterflies, was a just object of ridicule, and we must give him up
as fair game to the satyrist.
Adieu, my dear friend, and believe me ever
Yours affectionately,
B. FRANKLIN.
TO THE SAME.
_On the Bristol Waters, and the Tide in Rivers._
_London, Sept. 13, 1760._
MY DEAR FRIEND,
I have your agreeable letter from Bristol, which I take this first
leisure hour to answer, having for some time been much engaged in
business.
Your first question, _What is the reason the water at this place,
though cold at the spring, becomes warm by pumping?_ It will be
most prudent in me to forbear attempting to answer, till, by a more
circumstantial account, you assure me of the fact. I own I should
expect that operation to warm, not so much the water pumped, as the
person pumping.--The rubbing of dry solids together has been long
observed to produce heat; but the like effect has never yet, that
I have heard, been produced by the mere agitation of fluids, or
friction of fluids with solids. Water in a bottle shook for hours
by a mill-hopper, it is said, discovered no sensible addition of
heat. The production of animal heat by exercise is therefore to be
accounted for in another manner, which I may hereafter endeavour to
make you acquainted with.
This prudence of not attempting to give reasons before one is sure of
facts, I learnt from one of your sex, who, as Selden tells us, being
in company with some gentlemen that were viewing, and considering
something which they called a Chinese shoe, and disputing earnestly
about the manner of wearing it, and how it could possibly be put on;
put in her word, and said modestly, _Gentlemen, are you sure it is a
shoe?--Should not that be settled first?_
But I shall now endeavour to explain what I said to you about the
tide in rivers, and to that end shall make a figure, which though
not very like a river, may serve to convey my meaning.--Suppose a
canal one hundred and forty miles long, communicating at one end with
the sea, and filled therefore with sea water. I chuse a canal at
first, rather than a river, to throw out of consideration the effects
produced by the streams of fresh water from the land, the inequality
in breadth, and the crookedness of courses.
[Illustration: (showing tidal wave effects)]
Let A, C, be the head of the canal; C, D, the bottom of it; D, F,
the open mouth of it next the sea. Let the strait pricked line, B,
G, represent low water mark the whole length of the canal, A, F,
high water mark:--Now if a person standing at E, and observing at
the time of high water there, that the canal is quite full at that
place up to the line E, should conclude that the canal is equally
full to the same height from end to end, and therefore there was as
much more water come into the canal since it was down at low water
mark, as would be included in the oblong space A, B, G, F, he would
be greatly mistaken. For the tide is _a wave_, and the top of the
wave, which makes high water, as well as every other lower part, is
progressive; and it is high water successively, but not at the same
time, in all the several points between G, F, and A, B.--And in such
a length as I have mentioned it is low water at F, G, and also at A,
B, at or near the same time with its being high water at E; so that
the surface of the water in the canal, during that situation, is
properly represented by the curve pricked line B, E, G. And on the
other hand, when it is low water at E, H, it is high water both at F,
G, and at A, B, at or near the same time: and the surface would then
be described by the inverted curve line, A, H, F.
In this view of the case, you will easily see, that there must be
very little more water in the canal at what we call high water, than
there is at low water, those terms not relating to the whole canal at
the same time, but successively to its parts. And if you suppose the
canal six times as long, the case would not vary as to the quantity
of water at different times of the tide; there would only be six
waves in the canal at the same time, instead of one, and the hollows
in the water would be equal to the hills.
That this is not mere theory, but conformable to fact, we know by
our long rivers in America. The Delaware, on which Philadelphia
stands, is in this particular similar to the canal I have supposed
of one wave: for when it is high water at the Capes or mouth of the
river, it is also high water at Philadelphia, which stands about one
hundred and forty miles from the sea; and there is at the same time
a low water in the middle between the two high waters; where, when
it comes to be high water, it is at the same time low water at the
Capes and at Philadelphia. And the longer rivers have some a wave
and half, some two, three, or four waves, according to their length.
In the shorter rivers of this island, one may see the same thing in
part: for instance, it is high water at Gravesend an hour before it
is high water at London Bridge; and twenty miles below Gravesend an
hour before it is high water at Gravesend. Therefore at the time of
high water at Gravesend the top of the wave is there, and the water
is then not so high by some feet where the top of the wave was an
hour before, or where it will be an hour after, as it is just then at
Gravesend.
Now we are not to suppose, that because the swell or top of the
wave runs at the rate of twenty miles an hour, that therefore the
current, or water itself of which the wave is composed, runs at that
rate. Far from it. To conceive this motion of a wave, make a small
experiment or two. Fasten one end of a cord in a window near the top
of a house, and let the other end come down to the ground; take this
end in your hand, and you may, by a sudden motion, occasion a wave in
the cord that will run quite up to the window; but though the wave
is progressive from your hand to the window, the parts of the rope do
not proceed with the wave, but remain where they were, except only
that kind of motion that produces the wave. So if you throw a stone
into a pond of water when the surface is still and smooth, you will
see a circular wave proceed from the stone as its centre, quite to
the sides of the pond; but the water does not proceed with the wave,
it only rises and falls to form it in the different parts of its
course; and the waves that follow the first, all make use of the same
water with their predecessors.
But a wave in water is not indeed in all circumstances exactly like
that in a cord; for water being a fluid, and gravitating to the
earth, it naturally runs from a higher place to a lower; therefore
the parts of the wave in water do actually run a little both ways
from its top towards its lower sides, which the parts of the wave
in the cord cannot do. Thus, when it is high and standing water at
Gravesend, the water twenty miles below has been running ebb, or
towards the sea for an hour, or ever since it was high water there;
but the water at London Bridge will run flood, or from the sea yet
another hour, till it is high water, or the top of the wave arrives
at that bridge, and then it will have run ebb an hour at Gravesend,
&c. &c. Now this motion of the water, occasioned only by its gravity,
or tendency to run from a higher place to a lower, is by no means so
swift as the motion of the wave. It scarce exceeds perhaps two miles
in an hour.
If it went as the wave does twenty miles an hour, no ships could
ride at anchor in such a stream, nor boats row against it.
In common speech, indeed, this current of the water both ways from
the top of the wave is called _the tide_; thus we say, _the tide runs
strong_, _the tide runs at the rate of one, two, or three miles an
hour, &c._ and when we are at a part of the river behind the top of
the wave, and find the water lower than high-water mark, and running
towards the sea, we say, _the tide runs ebb_; and when we are before
the top of the wave, and find the water higher than low-water mark,
and running from the sea, we say, _the tide runs flood_; but these
expressions are only locally proper; for a tide, strictly speaking,
is _one whole wave_, including all its parts higher and lower, and
these waves succeed one another about twice in twenty-four hours.
This motion of the water, occasioned by its gravity, will explain to
you why the water near the mouths of rivers may be salter at high
water than at low. Some of the salt-water, as the tide wave enters
the river, runs from its top and fore side, and mixes with the fresh,
and also pushes it back up the river.
Supposing that the water commonly runs during the flood at the rate
of two miles in an hour, and that the flood runs five hours, you see
that it can bring at most into our canal only a quantity of water
equal to the space included in the breadth of the canal, ten miles
of its length, and the depth between low and high-water mark; which
is but a fourteenth part of what would be necessary to fill all the
space between low and high-water mark, for one hundred and forty
miles, the whole length of the canal.
And indeed such a quantity of water as would fill that whole space,
to run in and out every tide, must create so outrageous a current, as
would do infinite damage to the shores, shipping, &c. and make the
navigation of a river almost impracticable.
I have made this letter longer than I intended, and therefore
reserve for another what I have further to say on the subject of
tides and rivers. I shall now only add, that I have not been exact
in the numbers, because I would avoid perplexing you with minute
calculations, my design at present being chiefly to give you distinct
and clear ideas of the first principles.
After writing six folio pages of philosophy to a young girl, is it
necessary to finish such a letter with a compliment?--Is not such
a letter of itself a compliment?--Does it not say, she has a mind
thirsty after knowledge, and capable of receiving it; and that the
most agreeable things one can write to her are those that tend to
the improvement of her understanding?--It does indeed say all this,
but then it is still no compliment; it is no more than plain honest
truth, which is not the character of a compliment. So if I would
finish my letter in the _mode_, I should yet add some thing that
means nothing, and is _merely_ civil and polite. But being naturally
aukward at every circumstance of ceremony, I shall not attempt it.
I had rather conclude abruptly with what pleases me more than any
compliment can please you, that I am allowed to subscribe myself
Your affectionate friend,
B. FRANKLIN.
TO THE SAME.
_On the same Subject._
_Craven-street, Monday, March 30, 1761._
MY DEAR FRIEND,
Supposing the fact, that the water of the well at Bristol is warmer
after sometime pumping, I think your manner of accounting for that
increased warmth very ingenious and probable. It did not occur to me,
and therefore I doubted of the fact.
You are, I think quite right in your opinion, that the rising of the
tides in rivers is not owing to the immediate influence of the moon
on the rivers. It is rather a subsequent effect of the influence
of the moon on the sea, and does not make its appearance in some
rivers till the moon has long passed by. I have not expressed myself
clearly if you have understood me to mean otherwise. You know I
have mentioned it as a fact, that there are in some rivers several
tides all existing at the same time; that is, two, three, or more,
high-waters, and as many low-waters, in different parts of the same
river, which cannot possibly be all effects of the moon's immediate
action on that river; but they may be subsequent effects of her
action on the sea.