good ladies your aunts, and to Miss Pitt, and believe me ever
B. FRANKLIN.
* * * * *
_To the same._
TENDENCY OF RIVERS TO THE SEA.--EFFECTS OF THE SUN'S RAYS ON CLOTHES OF
DIFFERENT COLOURS.
September 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 Chesapeake 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 bluish 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, how 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 sometimes 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 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
Chesapeake, in Virginia, mentioned above; you will see, communicating
with it by their mouths, the great rivers Susquehanna, Potomac,
Rappahannoc, 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 Susquehanna 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
a bright silver tankard.
My experiment was this. I took a number of little pieces of broadcloth
from a tailor'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
sunshiny 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 a uniform of
white? That summer hats for men or women should be white, as repelling
that heat which gives headaches 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 upward 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 daytime 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.
B. FRANKLIN.
* * * * *
_To the same._
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 the mercury in
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 observations 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 shipbuilding at the king's
yard in that country, a kind of worms was 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 inquire 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 the 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 satirist.
B. FRANKLIN.
* * * * *
_To Dr. Joseph Priestley._
EFFECT OF VEGETATION ON NOXIOUS AIR.
* * That the vegetable creation should restore the air which is spoiled
by the animal part of it, looks like a rational system, and seems to be
of a piece with the rest. Thus fire purifies water all the world over.
It purifies it by distillation, when it raises it in vapours, and lets
it fall in rain; and farther still by filtration, when, keeping it
fluid, it suffers that rain to percolate the earth. We knew before that
putrid animal substances were converted into sweet vegetables when mixed
with the earth and applied as manure; and now, it seems, that the same
putrid substances, mixed with the air, have a similar effect. The
strong, thriving state of your mint, in putrid air, seems to show that
the air is mended by taking something from it, and not by adding to it.
I hope this will give some check to the rage of destroying trees that
grow near houses, which has accompanied our late improvements in
gardening, from an opinion of their being unwholesome. I am certain,
from long observation, that there is nothing unhealthy in the air of
woods; for we Americans have everywhere our country habitations in the
midst of woods, and no people on earth enjoy better health or are more
prolific.
B. FRANKLIN.
* * * * *
_To Dr. John Pringle._
ON THE DIFFERENCE OF NAVIGATION IN SHOAL AND DEEP WATER.
Craven-street, May 10, 1768.
You may remember, that when we were travelling together in Holland, you
remarked that the trackschuyt in one of the stages went slower than
usual, and inquired of the boatman what might be the reason; who
answered, that it had been a dry season, and the water in the canal was
low. On being asked if it was so low as that the boat touched the muddy
bottom, he said no, not so low as that, but so low as to make it harder
for the horse to draw the boat. We neither of us, at first, could
conceive, that if there was water enough for the boat to swim clear of
the bottom, its being deeper would make any difference; but as the man
affirmed it seriously as a thing well known among them, and as the
punctuality required in their stages was likely to make such difference,
if any there were, more readily observed by them than by other watermen
who did not pass so regularly and constantly backward and forward in the
same track, I began to apprehend there might be something in it, and
attempted to account for it from this consideration, that the boat, in
proceeding along the canal, must in every boat's length of her course
move out of her way a body of water equal in bulk to the room her bottom
took up in the water; that the water so moved must pass on each side of
her and under her bottom to get behind her; that if the passage under
her bottom was straitened by the shallows, more of that water must pass
by her sides, and with a swifter motion, which would retard her, as
moving the contrary way; or, that the water becoming lower behind the
boat than before, she was pressed back by the weight of its difference
in height, and her motion retarded by having that weight constantly to
overcome. But as it is often lost time to attempt accounting for
uncertain facts, I determined to make an experiment of this when I
should have convenient time and opportunity.
After our return to England, as often as I happened to be on the Thames,
I inquired of our watermen whether they were sensible of any difference
in rowing over shallow or deep water. I found them all agreeing in the
fact, that there was a very great difference, but they differed widely
in expressing the quantity of the difference; some supposing it was
equal to a mile in six, others to a mile in three, &c. As I did not
recollect to have met with any mention of this matter in our
philosophical books, and conceiving that if the difference should really
be great, it might be an object of consideration in the many projects
now on foot for digging new navigable canals in this island, I lately
put my design of making the experiment in execution in the following
manner.
I provided a trough of planed boards fourteen feet long, six inches
wide, and six inches deep in the clear, filled with water within half an
inch of the edge, to represent a canal. I had a loose board, of nearly
the same length and breadth, that, being put into the water, might be
sunk to any depth, and fixed by little wedges where I would choose to
have it stay, in order to make different depths of water, leaving the
surface at the same height with regard to the sides of the trough. I had
a little boat in form of a lighter or boat of burden, six inches long,
two inches and a quarter wide, and one inch and a quarter deep. When
swimming, it drew one inch water. To give motion to the boat, I fixed
one end of a long silk thread to its bow, just even with the water's
edge; the other end passed over a well-made brass pully, of about an
inch diameter, turning freely on a small axis; and a shilling was the
weight. Then placing the boat at one end of the trough, the weight
would draw it through the water to the other.
Not having a watch that shows seconds, in order to measure the time
taken up by the boat in passing from end to end, I counted as fast as I
could count to ten repeatedly, keeping an account of the number of tens
on my fingers. And as much as possible to correct any little
inequalities in my counting, I repeated the experiment a number of times
at each depth of water, that I might take the medium. And the following
are the results:
Water
1-1/2 inches deep. 2 inches. 4-1/2 inches.
1st exp. 100 94 79
2d " 104 93 78
3d " 104 91 77
4th " 106 87 79
5th " 100 88 79
6th " 99 86 80
7th " 100 90 79
8th " 100 88 81
--- --- ---
813 717 632
--- --- ---
Medium 101 Medium 89 Medium 79
I made many other experiments, but the above are those in which I was
most exact; and they serve sufficiently to show that the difference is
considerable. Between the deepest and shallowest it appears to be
somewhat more than one fifth. So that, supposing large canals, and
boats, and depths of water to bear the same proportions, and that four
men or horses would draw a boat in deep water four leagues in four
hours, it would require five to draw the same boat in the same time as
far in shallow water, or four would require five hours.
Whether this difference is of consequence enough to justify a greater
expense in deepening canals, is a matter of calculation, which our
ingenious engineers in that way will readily determine.
B. FRANKLIN.
* * * * *
_To Oliver Neale._
ON THE ART OF SWIMMING.
I cannot be of opinion with you, that it is too late in life for you to
learn to swim. The river near the bottom of your garden affords a most
convenient place for the purpose. And as your new employment requires
your being often on the water, of which you have such a dread, I think
you would do well to make the trial; nothing being so likely to remove
those apprehensions as the consciousness of an ability to swim to the
shore in case of an accident, or of supporting yourself in the water
till a boat could come to take you up.
I do not know how far corks or bladders may be useful in learning to
swim, having never seen much trial of them. Possibly they may be of
service in supporting the body while you are learning what is called the
stroke, or that manner of drawing in and striking out the hands and feet
that is necessary to produce progressive motion. But you will be no
swimmer till you can place some confidence in the power of the water to
support you; I would therefore advise the acquiring that confidence in
the first place, especially as I have known several who, by a little of
the practice necessary for that purpose, have insensibly acquired the
stroke, taught, as it were, by nature.
The practice I mean is this. Choosing a place where the water deepens
gradually, walk coolly into it till it is up to your breast; then turn
round, your face to the shore, and throw an egg into the water between
you and the shore. It will sink to the bottom, and be easily seen there,
as your water is clear. It must lie in water so deep as that you cannot
reach it to take it up but by diving for it. To encourage yourself in
order to do this, reflect that your progress will be from deeper to
shallower water, and that at any time you may, by bringing your legs
under you and standing on the bottom, raise your head far above the
water. Then plunge under it with your eyes open, throwing yourself
towards the egg, and endeavouring, by the action of your hands and feet
against the water, to get forward till within reach of it. In this
attempt you will find that the water buoys you up against your
inclination; that it is not so easy a thing to sink as you imagined;
that you cannot, but by active force, get down to the egg. Thus you feel
the power of the water to support you, and learn to confide in that
power; while your endeavours to overcome it and to reach the egg teach
you the manner of acting on the water with your feet and hands, which
action is afterward used in swimming to support your head higher above
water, or to go forward through it.
I would the more earnestly press you to the trial of this method,
because, though I think I satisfied you that your body is lighter than
water, and that you might float in it a long time, with your mouth free
for breathing, if you would put yourself in a proper posture, and would
be still and forbear struggling, yet, till you have obtained this
experimental confidence in the water, I cannot depend on your having the
necessary presence of mind to recollect that posture and directions I
gave you relating to it. The surprise may put all out of your mind. For
though we value ourselves on being reasonable, knowing creatures, reason
and knowledge seem, on such occasions, to be of little use to us; and
the brutes, to whom we allow scarce a glimmering of either, appear to
have the advantage of us.
I will, however, take this opportunity of repeating those particulars to
you which I mentioned in our last conversation, as, by perusing them at
your leisure, you may possibly imprint them so in your memory as, on
occasion, to be of some use to you.
1. That though the legs, arms, and head of a human body, being solid
parts, are specifically something heavier than fresh water, yet the
trunk, particularly the upper part, from its hollowness, is so much
lighter than water, as that the whole of the body, taken together, is
too light to sink wholly under water, but some part will remain above
until the lungs become filled with water, which happens from drawing
water into them instead of air, when a person, in the fright, attempts
breathing while the mouth and nostrils are under water.
2. That the legs and arms are specifically lighter than salt water, and
will be supported by it, so that a human body would not sink in salt
water, though the lungs were filled as above, but from the greater
specific gravity of the head.
3. That, therefore, a person throwing himself on his back in salt water,
and extending his arms, may easily lie so as to keep his mouth and
nostrils free for breathing; and, by a small motion of his hands, may
prevent turning if he should perceive any tendency to it.
4. That in fresh water, if a man throws himself on his back near the
surface, he cannot long continue in that situation but by proper action
of his hands on the water. If he uses no such action, the legs and lower
part of the body will gradually sink till he comes into an upright
position, in which he will continue suspended, the hollow of the breast
keeping the head uppermost.