pond, which I observed one day to be very rough with the wind, I
fetched out a cruet of oil, and dropt a little of it on the water.
I saw it spread itself with surprising swiftness upon the surface;
but the effect of smoothing the waves was not produced; for I had
applied it first on the leeward side of the pond, where the waves
were largest, and the wind drove my oil back upon the shore. I
then went to the windward side where they began to form; and there
the oil, though not more than a tea spoonful, produced an instant
calm over a space several yards square, which spread amazingly,
and extended itself gradually till it reached the lee side, making
all that quarter of the pond, perhaps half an acre, as smooth as a
looking-glass.
After this I contrived to take with me, whenever I went into the
country, a little oil in the upper hollow joint of my bamboo cane,
with which I might repeat the experiment as opportunity should offer,
and I found it constantly to succeed.
In these experiments, one circumstance struck me with particular
surprise. This was the sudden, wide and forcible spreading of a drop
of oil on the face of the water, which I do not know that any body
has hitherto considered. If a drop of oil is put on a highly polished
marble table, or on a looking-glass that lies horizontally, the drop
remains in its place, spreading very little. But when put on water,
it spreads instantly many feet round, becoming so thin as to produce
the prismatic colours, for a considerable space, and beyond them so
much thinner as to be invisible, except in its effect of smoothing
the waves at a much greater distance. It seems as if a mutual
repulsion between its particles took place as soon as it touched the
water, and a repulsion so strong as to act on other bodies swimming
on the surface, as straw, leaves, chips, &c. forcing them to recede
every way from the drop, as from a centre, leaving a large clear
space. The quantity of this force, and the distance to which it
will operate, I have not yet ascertained; but I think it a curious
enquiry, and I wish to understand whence it arises.
In our journey to the north, when we had the pleasure of seeing you
at Ormathwaite, we visited the celebrated Mr. Smeaton, near Leeds.
Being about to show him the smoothing experiment on a little pond
near his house, an ingenious pupil of his, Mr. Jessop, then present,
told us of an odd appearance on that pond, which had lately occurred
to him. He was about to clean a little cup in which he kept oil, and
he threw upon the water some flies that had been drowned in the oil.
These flies presently began to move, and turned round on the water
very rapidly, as if they were vigorously alive, though on examination
he found they were not so. I immediately concluded that the motion
was occasioned by the power of the repulsion above mentioned, and
that the oil issuing gradually from the spungy body of the fly
continued the motion. He found some more flies drowned in oil, with
which the experiment was repeated before us. To show that it was not
any effect of life recovered by the flies, I imitated it by little
bits of oiled chips and paper cut in the form of a comma, of the size
of a common fly; when the stream of repelling particles issuing from
the point made the comma turn round the contrary way. This is not a
chamber experiment; for it cannot be well repeated in a bowl or dish
of water on a table. A considerable surface of water is necessary to
give room for the expansion of a small quantity of oil. In a dish of
water, if the smallest drop of oil be let fall in the middle, the
whole surface is presently covered with a thin greasy film proceeding
from the drop; but as soon as that film has reached the sides of the
dish, no more will issue from the drop, but it remains in the form
of oil, the sides of the dish putting a stop to its dissipation by
prohibiting the farther expansion of the film.
Our friend Sir John Pringle, being soon after in Scotland, learned
there, that those employed in the herring-fishery could at a distance
see where the shoals of herrings were, by the smoothness of the water
over them, which might possibly be occasioned, he thought, by some
oiliness proceeding from their bodies.
A gentleman from Rhode-island told me, it had been remarked, that the
harbour of Newport was ever smooth while any whaling vessels were
in it; which probably arose from hence, that the blubber which they
sometimes bring loose in the hold, or the leakage of their barrels,
might afford some oil, to mix with that water, which from time to
time they pump out to keep their vessel free, and that some oil might
spread over the surface of the water in the harbour, and prevent the
forming of any waves.
This prevention I would thus endeavour to explain.
There seems to be no natural repulsion between water and air, such as
to keep them from coming into contact with each other. Hence we find
a quantity of air in water; and if we extract it by means of the
air-pump, the same water, again exposed to the air, will soon imbibe
an equal quantity.
Therefore air in motion, which is wind, in passing over the smooth
surface of water, may rub, as it were, upon that surface, and raise
it into wrinkles, which, if the wind continues, are the elements of
future waves.
The smallest wave once raised does not immediately subside, and leave
the neighbouring water quiet: but in subsiding raises nearly as much
of the water next to it, the friction of the parts making little
difference. Thus a stone dropped in a pool raises first a single wave
round itself; and leaves it, by sinking to the bottom; but that first
wave subsiding raises a second, the second a third, and so on in
circles to a great extent.
A small power continually operating will produce a great action. A
finger applied to a weighty suspended bell can at first move it but
little; if repeatedly applied, though with no greater strength, the
motion increases till the bell swings to its utmost height, and with
a force that cannot be resisted by the whole strength of the arm and
body. Thus the small first-raised waves, being continually acted upon
by the wind, are, though the wind does not increase in strength,
continually increased in magnitude, rising higher and extending their
bases, so as to include a vast mass of water in each wave, which in
its motion acts with great violence.
But if there be a mutual repulsion between the particles of oil, and
no attraction between oil and water, oil dropped on water will not be
held together by adhesion to the spot whereon it falls; it will not
be imbibed by the water; it will be at liberty to expand itself; and
it will spread on a surface that, besides being smooth to the most
perfect degree of polish, prevents, perhaps by repelling the oil,
all immediate contact, keeping it at a minute distance from itself;
and the expansion will continue till the mutual repulsion between
the particles of the oil is weakened and reduced to nothing by their
distance.
Now I imagine that the wind, blowing over water thus covered with
a film of oil, cannot easily _catch_ upon it, so as to raise the
first wrinkles, but slides over it, and leaves it smooth as it finds
it. It moves a little the oil indeed, which being between it and
the water, serves it to slide with, and prevents friction, as oil
does between those parts of a machine, that would otherwise rub
hard together. Hence the oil dropped on the windward side of a pond
proceeds gradually to leeward, as may be seen by the smoothness it
carries with it, quite to the opposite side. For the wind being thus
prevented from raising the first wrinkles, that I call the elements
of waves, cannot produce waves, which are to be made by continually
acting upon, and enlarging those elements, and thus the whole pond is
calmed.
Totally therefore we might suppress the waves in any required place,
if we could come at the windward place where they take their rise.
This in the ocean can seldom if ever be done. But perhaps something
may be done on particular occasions, to moderate the violence of the
waves when we are in the midst of them, and prevent their breaking
where that would be inconvenient.
For when the wind blows fresh, there are continually rising on the
back of every great wave a number of small ones, which roughen its
surface, and give the wind hold, as it were, to push it with greater
force. This hold is diminished, by preventing the generation of
those small ones. And possibly too, when a wave's surface is oiled,
the wind, in passing over it, may rather in some degree press
it down, and contribute to prevent it, rising again, instead of
promoting it.
This as mere conjecture would have little weight, if the apparent
effects of pouring oil into the midst of waves were not considerable,
and as yet not otherwise accounted for.
When the wind blows so fresh, as that the waves are not sufficiently
quick in obeying its impulse, their tops being thinner and lighter
are pushed forward, broken, and turned over in a white foam. Common
waves lift a vessel without entering it; but these when large
sometimes break above and pour over it, doing great damage.
That this effect might in any degree be prevented, or the height
and violence of waves in the sea moderated, we had no certain
account; Pliny's authority for the practice of seamen in his time
being slighted. But discoursing lately on this subject with his
excellency Count Bentinck, of Holland, his son the honourable Captain
Bentinck, and the learned professor Allemand (to all whom I showed
the experiment of smoothing in a windy day the large piece of water
at the head of the Green Park) a letter was mentioned, which had
been received by the Count from Batavia, relative to the saving of a
Dutch ship in a storm by pouring oil into the sea. I much desired to
see that letter, and a copy of it was promised me, which I afterward
received.
FOOTNOTES:
[28] Note by Dr. Brownrigg.
Sir Gilfred Lawson, who served long in the army at Gibraltar,
assures me, that the fishermen in that place are accustomed to pour
a little oil on the sea, in order to still its motion, that they may
be enabled to see the oysters lying at its bottom; which are there
very large, and which they take up with a proper instrument. This
Sir Gilfred had often seen there performed, and said the same was
practised on other parts of the Spanish coast.
[29] See the preceding paper. _Editor._
_Extract of a Letter from Mr. Tengnagel to Count Bentinck, dated at
Batavia, the 5th of January, 1770._
"Near the islands Paul and Amsterdam, we met with a storm, which had
nothing particular in it worthy of being communicated to you, except
that the captain found himself obliged for greater safety in wearing
the ship, to pour oil into the sea, to prevent the waves breaking
over her, which had an excellent effect, and succeeded in preserving
us. As he poured out but a little at a time, the East India Company
owes perhaps its ship to only six demi-ames of oil-olive. I was
present upon deck when this was done; and I should not have mentioned
this circumstance to you, but that we have found people here so
prejudiced against the experiment, as to make it necessary for the
officers on board and myself to give a certificate of the truth on
this head, of which we made no difficulty."
On this occasion, I mentioned to Captain Bentinck, a thought
which had occurred to me in reading the voyages of our late
circumnavigators, particularly where accounts are given of pleasant
and fertile islands which they much desired to land upon, when
sickness made it more necessary, but could not effect a landing
through a violent surf breaking on the shore, which rendered it
impracticable. My idea was, that possibly by sailing to and fro at
some distance from such lee-shore, continually pouring oil into the
sea, the waves might be so much depressed, and lessened before they
reached the shore, as to abate the height and violence of the surf,
and permit a landing; which, in such circumstances, was a point of
sufficient importance to justify the expense of the oil that might
be requisite for the purpose. That gentleman, who is ever ready
to promote what may be of public utility, though his own ingenious
inventions have not always met with the countenance they merited, was
so obliging as to invite me to Portsmouth, where an opportunity would
probably offer, in the course of a few days, of making the experiment
on some of the shores about Spithead, in which he kindly proposed
to accompany me, and to give assistance with such boats as might be
necessary. Accordingly, about the middle of October last, I went with
some friends to Portsmouth; and a day of wind happening, which made
a lee-shore between Haslar-hospital and the point near Jillkecker,
we went from the Centaur with the long-boat and barge towards that
shore. Our disposition was this: the long-boat was anchored about a
quarter of a mile from the shore; part of the company were landed
behind the point (a place more sheltered from the sea) who came
round and placed themselves opposite to the long-boat, where they
might observe the surf, and note if any change occurred in it upon
using the oil. Another party, in the barge, plied to windward of the
long-boat, as far from her as she was from the shore, making trips
of about half a mile each, pouring oil continually out of a large
stone-bottle, through a hole in the cork, somewhat bigger than a
goose-quill. The experiment had not, in the main point, the success
we wished, for no material difference was observed in the height or
force of the surf upon the shore; but those who were in the long-boat
could observe a tract of smoothed water, the whole of the distance in
which the barge poured the oil, and gradually spreading in breadth
towards the long-boat. I call it smoothed, not that it was laid
level; but because, though the swell continued, its surface was not
roughened by the wrinkles, or smaller waves, before-mentioned; and
none or very few white caps (or waves whose tops turn over in foam)
appeared in that whole space, though to windward and leeward of it
there were plenty; and a wherry, that came round the point under
sail, in her way to Portsmouth, seemed to turn into that tract of
choice, and to use it from end to end, as a piece of turnpike-road.
It may be of use to relate the circumstances of an experiment that
does not succeed, since they may give hints of amendment in future
trials: it is therefore I have been thus particular. I shall only add
what I apprehend may have been the reason of our disappointment.
I conceive, that the operation of oil on water is, first, to prevent
the raising of new waves by the wind; and, secondly, to prevent its
pushing those before raised with such force, and consequently their
continuance of the same repeated height, as they would have done, if
their surface were not oiled. But oil will not prevent waves being
raised by another power, by a stone, for instance, falling into a
still pool; for they then rise by the mechanical impulse of the
stone, which the greasiness on the surrounding water cannot lessen
or prevent, as it can prevent the winds catching the surface and
raising it into waves. Now waves once raised, whether by the wind or
any other power, have the same mechanical operation, by which they
continue to rise and fall, as a _pendulum_ will continue to swing,
a long time after the force ceases to act by which the motion was
first produced: that motion will, however, cease in time; but time
is necessary. Therefore, though oil spread on an agitated sea may
weaken the push of the wind on those waves whose surfaces are covered
by it, and so, by receiving less fresh impulse, they may gradually
subside; yet a considerable time, or a distance through which they
will take time to move, may be necessary to make the effect sensible
on any shore in a diminution of the surf: for we know, that when
wind ceases suddenly, the waves it has raised do not as suddenly
subside, but settle gradually, and are not quite down till after the
wind has ceased. So though we should, by oiling them, take off the
effect of wind on waves already raised, it is not to be expected
that those waves should be instantly levelled. The motion they have
received will, for some time, continue; and if the shore is not far
distant, they arrive there so soon, that their effect upon it will
not be visibly diminished. Possibly, therefore, if we had begun our
operations at a greater distance, the effect might have been more
sensible. And perhaps we did not pour oil in sufficient quantity.
Future experiments may determine this.
I was, however, greatly obliged to Captain Bentinck, for the chearful
and ready aids he gave me: and I ought not to omit mentioning Mr.
Banks, Dr. Solander, General Carnoc, and Dr. Blagden, who all
assisted at the experiment, during that blustering unpleasant day,
with a patience and activity that could only be inspired by a zeal
for the improvement of knowledge, such especially as might possibly
be of use to men in situations of distress.
I would wish you to communicate this to your ingenious friend, Mr.
Farish, with my respects; and believe me to be, with sincere esteem,
Dear Sir,
Your most obedient, humble servant,
B. FRANKLIN.
TO SIR JOHN PRINGLE, BART.
_On the Difference of Navigation in shoal and deep Water._
_Craven-street, May 10, 1768._
SIR,
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 enquired 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 again 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 backwards and forwards 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 enquired 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 plained 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
chuse 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
burthen, 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 1½ inches deep. 2 inches. 4½ inches.
1st exp. 100 94 79
2 104 93 78
3 104 91 77
4 106 87 79
5 100 88 79
6 99 86 80
7 100 90 79
8 100 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
expence in deepening canals, is a matter of calculation, which our
ingenious engineers in that way will readily determine. I am, &c.
B. FRANKLIN.
TO MR. ALPHONSUS LE ROY, MEMBER OF SEVERAL ACADEMIES AT PARIS.[30]
_Containing sundry Maritime Observations._
At Sea, on board the London Packet, Capt. Truxton.
_Aug. 1785._
SIR,
Your learned writings on the navigation of the antients, which
contain a great deal of curious information, and your very ingenious
contrivances for improving the modern sails (_voilure_) of which
I saw with great pleasure a successful trial on the river Seine,
have induced me to submit to your consideration and judgment, some
thoughts I have had on the latter subject.
Those mathematicians, who have endeavoured to improve the swiftness
of vessels, by calculating to find the form of least resistance, seem
to have considered a ship as a body moving through one fluid only,
the water; and to have given little attention to the circumstance
of her moving through another fluid, the air. It is true that when
a vessel sails right before the wind, this circumstance is of no
importance, because the wind goes with her; but in every deviation
from that course, the resistance of the air is something, and becomes
greater in proportion as that deviation increases. I wave at present
the consideration of those different degrees of resistance given by
the air to that part of the hull which is above water, and confine
myself to that given to the sails; for their motion through the air
is resisted by the air, as the motion of the hull through the
water is resisted by the water, though with less force as the air is
a lighter fluid. And to simplify the discussion as much as possible,
I would state one situation only, to wit, that of the wind upon the
beam, the ship's course being directly across the wind; and I would
suppose the sail set in an angle of 45 degrees with the keel, as in
the following figure; wherein (Plate VI, Fig. 1.)
[Illustration:
_Plate VI._ MARITIME OBSERVATIONS. _Vol. II. page 163._
_Published as the Act directs, April 1, 1806, by Longman, Hurst, Rees
& Orme, Paternoster Row._]