relating to the air, and the evaporation of water. It is Mr.
Collinson's copy, who took it from one I sent through his hands to a
correspondent in France some years since; I have, as he desired me,
corrected the mistakes he made in transcribing, and must return it to
him; but if you think it worth while, you may take a copy of it: I
would have saved you any trouble of that kind, but had not time.
Some day in the next or the following week, I purpose to have the
pleasure of seeing you at Wanstead: I shall accompany your good mamma
thither, and stay till the next morning, if it may be done without
incommoding your family too much.--We may then discourse any points
in that paper that do not seem clear to you; and taking a walk to
lord Tilney's ponds, make a few experiments there to explain the
nature of the tides more fully. In the mean time, believe me to be,
with the highest esteem and regard, your sincerely affectionate
friend,
B. FRANKLIN.
TO THE SAME.
_Salt-Water rendered fresh by Distillation.--Method of relieving
Thirst by Sea-Water._
_Craven-street, August 10, 1761._
We are to set out this week for Holland, where we may possibly spend
a month, but purpose to be at home again before the coronation. I
could not go without taking leave of you by a line at least, when I
am so many letters in your debt.
In yours of May 19, which I have before me, you speak of the ease
with which salt water may be made fresh by distillation, supposing it
to be, as I had said, that in evaporation the air would take up water
but not the salt that was mixed with it. It is true that distilled
sea water will not be salt, but there are other disagreeable
qualities that rise with the water in distillation; which indeed
several besides Dr. Hales have endeavoured by some means to prevent;
but as yet their methods have not been brought much into use.
I have a singular opinion on this subject, which I will venture to
communicate to you, though I doubt you will rank it among my whims.
It is certain that the skin has _imbibing_ as well as _discharging_
pores; witness the effects of a blistering plaister, &c. I have read
that a man, hired by a physician to stand by way of experiment in
the open air naked during a moist night, weighed near three pounds
heavier in the morning. I have often observed myself, that however
thirsty I may have been before going into the water to swim, I am
never long so in the water. These imbibing pores, however, are very
fine, perhaps fine enough in filtering to separate salt from water;
for though I have soaked (by swimming, when a boy) several hours in
the day for several days successively in salt-water, I never found
my blood and juices salted by that means, so as to make me thirsty
or feel a salt taste in my mouth: and it is remarkable, that the
flesh of sea fish, though bred in salt-water, is not salt.--Hence I
imagine, that if people at sea, distressed by thirst when their fresh
water is unfortunately spent, would make bathing-tubs of their empty
water-casks, and, filling them with sea-water, sit in them an hour or
two each day, they might be greatly relieved. Perhaps keeping their
clothes constantly wet might have an almost equal effect; and this
without danger of catching cold. Men do not catch cold by wet cloaths
at sea. Damp, but not wet linen may possibly give colds; but no one
catches cold by bathing, and no clothes can be wetter than water
itself. Why damp clothes should then occasion colds, is a curious
question, the discussion of which I reserve for a future letter, or
some future conversation.
Adieu, my little philosopher. Present my respectful compliments to
the good ladies your aunts, and to miss Pitt; and believe me ever
Your affectionate friend,
And humble Servant,
B. FRANKLIN.
TO THE SAME.
_Tendency of Rivers to the Sea.--Effect of the Sun's Rays on Cloths
of different Colours._
_Sept. 20, 1761._
MY DEAR FRIEND,
It is, as you observed in our late conversation, a very general
opinion, that _all rivers run into the sea_, or deposite their
waters there. 'Tis a kind of audacity to call such general opinions
in question, and may subject one to censure. But we must hazard
something in what we think the cause of truth: and if we propose our
objections modestly, we shall, though mistaken, deserve a censure
less severe, than when we are both mistaken and insolent.
That some rivers run into the sea is beyond a doubt: such, for
instance, are the Amazons, and I think the Oronoko and the
Mississippi. The proof is, that their waters are fresh quite to the
sea, and out to some distance from the land. Our question is, whether
the fresh waters of those rivers whose beds are filled with salt
water to a considerable distance up from the sea (as the Thames,
the Delaware, and the rivers that communicate with Chesapeak-bay in
Virginia) do ever arrive at the sea? And as I suspect they do not, I
am now to acquaint you with my reasons; or, if they are not allowed
to be reasons, my conceptions at least, of this matter.
The common supply of rivers is from springs, which draw their origin
from rain that has soaked into the earth. The union of a number of
springs forms a river. The waters as they run, exposed to the sun,
air, and wind, are continually evaporating. Hence in travelling one
may often see where a river runs, by a long blueish mist over it,
though we are at such a distance as not to see the river itself. The
quantity of this evaporation is greater or less, in proportion to
the surface exposed by the same quantity of water to those causes of
evaporation. While the river runs in a narrow confined channel in the
upper hilly country, only a small surface is exposed; a greater as
the river widens. Now if a river ends in a lake, as some do, whereby
its waters are spread so wide as that the evaporation is equal to
the sum of all its springs, that lake will never overflow:--And if
instead of ending in a lake, it was drawn into greater length as a
river, so as to expose a surface equal in the whole to that lake, the
evaporation would be equal, and such river would end as a canal; when
the ignorant might suppose, as they actually do in such cases, that
the river loses itself by running under ground, whereas in truth it
has run up into the air.
Now, many rivers that are open to the sea widen much before they
arrive at it, not merely by the additional waters they receive,
but by having their course stopped by the opposing flood-tide; by
being turned back twice in twenty-four hours, and by finding broader
beds in the low flat countries to dilate themselves in; hence the
evaporation of the fresh water is proportionably increased; so that
in some rivers it may equal the springs of supply. In such cases,
the salt water comes up the river, and meets the fresh in that part
where, if there were a wall or bank of earth across from side to
side, the river would form a lake, fuller indeed at some times than
at others, according to the seasons, but whose evaporation would, one
time with another, be equal to its supply.
When the communication between the two kinds of water is open, this
supposed wall of separation may be conceived as a moveable one, which
is not only pushed some miles higher up the river by every flood tide
from the sea, and carried down again as far by every tide of ebb,
but which has even this space of vibration removed nearer to the sea
in wet seasons, when the springs and brooks in the upper country are
augmented by the falling rains, so as to swell the river, and farther
from the sea in dry seasons.
Within a few miles above and below this moveable line of separation,
the different waters mix a little, partly by their motion to and fro,
and partly from the greater specific gravity of the salt water, which
inclines it to run under the fresh, while the fresh water, being
lighter, runs over the salt.
Cast your eye on the map of North America, and observe the bay of
Chesapeak in Virginia, mentioned above; you will see, communicating
with it by their mouths, the great rivers Sasquehanah, Potowmack,
Rappahanock, York, and James, besides a number of smaller streams,
each as big as the Thames. It has been proposed by philosophical
writers, that to compute how much water any river discharges into the
sea in a given time, we should measure its depth and swiftness at any
part above the tide; as, for the Thames, at Kingston or Windsor. But
can one imagine, that if all the water of those vast rivers went to
the sea, it would not first have pushed the salt water out of that
narrow-mouthed bay, and filled it with fresh?--The Sasquehanah alone
would seem to be sufficient for this, if it were not for the loss by
evaporation. And yet that bay is salt quite up to Annapolis.
As to our other subject, the different degrees of heat imbibed from
the sun's rays by cloths of different colours, since I cannot find
the notes of my experiment to send you, I must give it as well as I
can from memory.
But first let me mention an experiment you may easily make yourself.
Walk but a quarter of an hour in your garden when the sun shines,
with a part of your dress white, and a part black; then apply your
hand to them alternately, and you will find a very great difference
in their warmth. The black will be quite hot to the touch, the white
still cool.
Another. Try to fire the paper with a burning glass. If it is white,
you will not easily burn it;--but if you bring the focus to a black
spot, or upon letters, written or printed, the paper will immediately
be on fire under the letters.
Thus fullers and dyers find black cloths, of equal thickness with
white ones, and hung out equally wet, dry in the sun much sooner
than the white, being more readily heated by the sun's rays. It is
the same before a fire; the heat of which sooner penetrates black
stockings than white ones, and so is apt sooner to burn a man's
shins. Also beer much sooner warms in a black mug set before the
fire, than in a white one, or in a bright silver tankard.
My experiment was this. I took a number of little square pieces of
broad cloth from a taylor's pattern-card, of various colours. There
were black, deep blue, lighter blue, green, purple, red, yellow,
white, and other colours, or shades of colours. I laid them all out
upon the snow in a bright sun-shiny morning. In a few hours (I cannot
now be exact as to the time) the black, being warmed most by the sun,
was sunk so low as to be below the stroke of the sun's rays; the dark
blue almost as low, the lighter blue not quite so much as the dark,
the other colours less as they were lighter; and the quite white
remained on the surface of the snow, not having entered it at all.
What signifies philosophy that does not apply to some use?---May we
not learn from hence, that black clothes are not so fit to wear in a
hot sunny climate or season, as white ones; because in such clothes
the body is more heated by the sun when we walk abroad, and are at
the same time heated by the exercise, which double heat is apt to
bring on putrid dangerous fevers? That soldiers and seamen, who must
march and labour in the sun, should in the East or West Indies have
an uniform of white? That summer hats, for men or women, should be
white, as repelling that heat which gives head-achs to many, and to
some the fatal stroke that the French call the _coup de soleil?_ That
the ladies summer hats, however, should be lined with black, as not
reverberating on their faces those rays which are reflected upwards
from the earth or water? That the putting a white cap of paper or
linen _within_ the crown of a black hat, as some do, will not keep
out the heat, though it would if placed _without_. That fruit-walls
being blacked may receive so much heat from the sun in the day-time,
as to continue warm in some degree through the night, and thereby
preserve the fruit from frosts, or forward its growth?--with sundry
other particulars of less or greater importance, that will occur from
time to time to attentive minds?--I am,
Yours affectionately,
B. FRANKLIN.
TO MR. HOPKINSON.
_On the Vis Inertiæ of Matter._
_Philadelphia, 1748._
SIR,
According to my promise, I send you _in writing_ my observations on
your book[18]: you will be the better able to consider them; which
I desire you to do at your leisure, and to set me right where I am
wrong.
I stumble at the threshold of the building, and therefore have not
read farther. The author's _vis inertiæ essential to matter_, upon
which the whole work is founded, I have not been able to comprehend.
And I do not think he demonstrates at all clearly (at least to me he
does not) that there is really _such a property in matter_.
He says, No. 2. "Let a given body or mass of matter be called
_a_, and let any given celerity be called c. That _celerity_
doubled, tripled, &c. or halved, thirded, &c. will be 2 _c_, 3 _c_,
&c. or ½ _c_, ⅓ _c_, &c. respectively: also the _body_ doubled,
tripled, or halved, thirded, will be 2 _a_, 3 _a_, or ½ _a_, ⅓ _a_,
respectively." Thus far is clear.--But he adds, "Now to move the body
_a_ with the celerity _c_, requires a certain force to be impressed
upon it; and to move it with a celerity as 2 _c_, requires _twice
that force_ to be impressed upon it, &c." Here I suspect some mistake
creeps in by the author's not distinguishing between a great force
applied at once, or a small one continually applied, to a mass of
matter, in order to move it. I think it is generally allowed by
the philosophers, and, for aught we know, is certainly true, that
there is no mass of matter, how great soever, but may be moved by
any force how small soever (taking friction out of the question)
and this small force continued, will in time bring the mass to move
with any velocity whatsoever.--Our author himself seems to allow
this towards the end of the same No. 2. when he is subdividing his
celerities and forces: for as in continuing the division to eternity
by his method of ½ _c_, ⅓ _c_, ¼ _c_, ⅕ _c_, &c. you can never come
to a fraction of velocity that is equal to 0 _c_, or no celerity at
all; so dividing the force in the same manner, you can never come
to a fraction of force that will not produce an equal fraction of
celerity.--Where then is the mighty vis inertiæ, and what is its
strength; when the greatest assignable mass of matter will give way
to, or be moved by the _least_ assignable force? Suppose two globes,
equal to the sun and to one another, exactly equipoised in Jove's
balance; suppose no friction in the centre of motion, in the beam or
elsewhere: if a musketo then were to light on one of them, would he
not give motion to them both, causing one to descend and the other
to rise? If it is objected, that the force of gravity helps one globe
to descend, I answer, the same force opposes the other's rising: here
is an equality that leaves the whole motion to be produced by the
musketo, without whom those globes would not be moved at all.--What
then does vis inertiæ do in this case? and what other effect could
we expect _if there were no such thing_? Surely if it were any thing
more than a phantom, there might be enough of it in such _vast_
bodies to annihilate, by its opposition to motion, so trifling a
force?
Our author would have reasoned more clearly, I think, if, as he has
used the letter _a_ for a certain quantity of matter, and _c_ for a
certain quantity of celerity, he had employed one letter more, and
put _f_ perhaps, for a certain quantity of force. This let us suppose
to be done; and then as it is a maxim that the force of bodies in
motion is equal to the quantity of matter multiplied by the celerity,
(or _f_ = _c_ X _a_); and as the force received by and subsisting in
matter, when it is put in motion, can never exceed the force given;
so if, _f_ moves _a_ with _c_, there must needs be required 2 _f_ to
move _a_ with 2 _c_; for _a_ moving with 2 _c_ would have a force
equal to 2 _f_, which it could not receive from 1 _f_; and this, not
because there is such a thing as vis inertiæ, for the case would be
the same _if that had no existence_; but because nothing _can_ give
more than it has, if 1 _f_ can to 1 _a_ give 1 _c_, which is the
same thing as giving it 1 _f_; (i. e. if force applied to matter at
rest, can put it in motion, and give it _equal_ force) where then is
vis inertiæ? If it existed at all in matter, should we not find the
quantity of its resistance subtracted from the force given?
In No. 4. our author goes on and says, "the body _a_ requires a
certain force to be impressed on it to be moved with a celerity as
_c_, or such a force is necessary; and therefore makes a certain
resistance, &c. A body as 2 _a_ requires _twice_ that force to be
moved with the _same celerity_, or it makes twice that resistance;
and so on."--This I think is not true; but that the body 2 _a_ moved
by the force 1 _f_ (though the eye may judge otherwise of it) does
really move with the same celerity as it did when impelled by the
same force; for 2 _a_ is compounded of 1 _a_ + 1 _a_: and if each of
the 1 _a_'s or each part of the compound were made to move with 1 _c_
(as they might be by 2 _f_) then the whole would move with 2 _c_, and
not with 1 _c_, as our author supposes. But 1 _f_ applied to 2 _a_,
makes each _a_ move with ½ _c_; and so the whole moves with 1 _c_;
exactly the same as 1 _a_ was made to do by 1 _f_ before. What is
equal celerity but a _measuring the same space by moving bodies in
the same time_?--Now if 1 _a_ impelled by 1 _f_ measures 100 yards
in a minute; and in 2 _a_ impelled by 1 _f_, each _a_ measures 50
yards in a minute, which added make 100; are not the celerities as
the forces equal? and since force and celerity in the same quantity
of matter are always in _proportion_ to each other, why should we,
when the quantity of matter is doubled, allow the force to continue
unimpaired, and yet suppose one half of the celerity to be lost?--I
wonder the more at our author's mistake in this point, since in the
same number I find him observing: "We may easily conceive that a body
as 3 _a_, 4 _a_, &c. would make 3 or 4 bodies equal to once _a_, each
of which would require once the first force to be moved with the
celerity _c_." If then in 3 _a_, each _a_ requires once the first
force _f_ to be moved with the celerity _c_, would not each move
with the force _f_ and celerity _c_; and consequently the whole be 3
_a_ moving with 3 _f_ and 3 _c?_ After so distinct an observation,
how could he miss of the consequence, and imagine that 1 _c_ and 3
_c_ were the same? Thus as our author's abatement of celerity in the
case of 2 _a_ moved by 1 _f_ is imaginary, so must be his additional
resistance.--And here again, I am at a loss to discover any effect of
the vis inertiæ.
In No. 6, he tells us, "that all this is likewise certain when taken
the contrary way, viz. _from motion to rest_; for the body _a_ moving
with a certain velocity, as _c_, requires a certain degree of force
or resistance to stop that motion, &c. &c." that is, in other words,
equal force is necessary to destroy force. It may be so. But how
does that discover a vis inertiæ? would not the effect be the same
_if there were no such thing_? A force 1 _f_ strikes a body 1 _a_,
and moves it with the celerity 1 _c_, i. e. with the force 1 _f_: It
requires, even according to our author, only an opposing 1 _f_ to
stop it. But ought it not (if there were a vis inertiæ) to have not
only the force 1 _f_, but an additional force equal to the force of
vis inertiæ, that _obstinate power by which a body endeavours with_
all its might _to continue in its present state, whether of motion
or rest_? I say, ought there not to be an opposing force equal to
the sum of these?--The truth however is, that there is no body, how
large soever, moving with any velocity, how great soever, but may be
stopped by any opposing force, how small soever, continually applied.
At least all our modern philosophers agree to tell us so.
Let me turn the thing in what light I please, I cannot discover the
vis inertiæ, nor any effect of it. It is allowed by all, that a body
1 _a_ moving with a velocity 1 _c_, and a force 1 _f_ _striking
another_ body 1 _a_ at rest, they will afterwards _move on together_,
each with ½ _c_ and ½ _f_; which, as I said before, is equal in the
whole to 1 _c_ and 1 _f_. If vis inertiæ, as in this case, neither
abates the force nor the velocity of bodies, what does it, or how
does it discover itself?
I imagine I may venture to conclude my observations on this piece,
almost in the words of the author; that if the doctrines of the
immateriality of the soul and the existence of God and of divine
providence are demonstrable from no plainer principles, the _deist_
[i.e. _theist_] has a desperate cause in hand. I oppose _my theist_
to his atheist, because I think they are diametrically opposite;
and not near of kin, as Mr. Whitfield seems to suppose; where (in
his journal) he tells us, "_M. B. was a deist, I had almost said an
atheist_;" that is, _chalk_, I had almost said _charcoal_.
The din of the market[19] increases upon me; and that, with frequent
interruptions, has, I find, made me say some things twice over; and,
I suppose, forget some others I intended to say. It has, however, one
good effect, as it obliges me to come to the relief of your patience
with
Your humble servant,
B. FRANKLIN.
FOOTNOTES:
[18] Baxter's Inquiry into the Nature of the Human Soul. B. V.
[19] Philadelphia market, in which Dr. Franklin lived. B. V.
TO JOHN PRINGLE, M. D. AND F. R. S.
_On the different Strata of the Earth._
_Craven-Street, Jan. 6, 1758._
SIR,
I return you Mr. Mitchell's paper on the strata of the earth[20] with
thanks. The reading of it, and perusal of the draft that accompanies
it, have reconciled me to those convulsions which all naturalists
agree this globe has suffered. Had the different strata of clay,
gravel, marble, coals, lime-stone, sand, minerals, &c. continued to
lie level, one under the other, as they may be supposed to have done
before those convulsions, we should have had the use only of a few
of the uppermost of the strata, the others lying too deep and too
difficult to be come at; but the shell of the earth being broke,
and the fragments thrown into this oblique position, the disjointed
ends of a great number of strata of different kinds are brought up
to day, and a great variety of useful materials put into our power,
which would otherwise have remained eternally concealed from us. So
that what has been usually looked upon as a _ruin_ suffered by this
part of the universe, was, in reality, only a preparation, or means
of rendering the earth more fit for use, more capable of being to
mankind a convenient and comfortable habitation.
I am, Sir, with great esteem, yours, &c.
B. FRANKLIN.
FOOTNOTE:
[20] See this paper afterwards printed in the _Philosophical
Transactions_.
TO THE ABBE SOULAVIE.
Occasioned by his sending me some notes he had taken of what I had
said to him in conversation on the Theory of the Earth. I wrote
it to set him right in some points wherein he had mistaken my
meaning.[21]
_Passy, September 22, 1782._
SIR,