EEE the direction of the wind, MM the line of motion. In observing
this figure it will appear, that so much of the body of the vessel
as is immersed in the water must, to go forward, remove out of its
way what water it meets with between the pricked lines FF. And the
sail, to go forward, must move out of its way all the air its whole
dimension meets with between the pricked lines CG and DG. Thus both
the fluids give resistance to the motion, each in proportion to the
quantity of matter contained in the dimension to be removed. And
though the air is vastly lighter than the water, and therefore more
easily removed, yet the dimension being much greater its effect is
very considerable.
It is true that in the case stated, the resistance given by the air
between those lines to the motion of the sail is not apparent to the
eye, because the greater force of the wind, which strikes it in the
direction EEE, overpowers its effect, and keeps the sail full in the
curve a, a, a, a, a. But suppose the wind to cease, and the vessel in
a calm to be impelled with the same swiftness by oars, the sail would
then appear filled in the contrary curve b, b, b, b, b, when prudent
men would immediately perceive, that the air resisted its motion,
and would order it to be taken in.
Is there any possible means of diminishing this resistance, while
the same quantity of sail is exposed to the action of the wind, and
therefore the same force obtained from it? I think there is, and
that it may be done by dividing the sail into a number of parts, and
placing those parts in a line one behind the other; thus instead of
one sail extending from C to D, figure 2, if four sails containing
together the same quantity of canvas, were placed as in figure 3,
each having one quarter of the dimensions of the great sail, and
exposing a quarter of its surface to the wind, would give a quarter
of the force; so that the whole force obtained from the wind would be
the same, while the resistance from the air would be nearly reduced
to the space between the pricked lines _a b_ and _c d_, before the
foremost sail.
It may perhaps be doubted whether the resistance from the air would
be so diminished; since possibly each of the following small sails
having also air before it, which must be removed, the resistance on
the whole would be the same.
This is then a matter to be determined by experiment. I will mention
one that I many years since made with success for another purpose;
and I will propose another small one easily made. If that too
succeeds, I should think it worth while to make a larger, though at
some expense, on a river boat; and perhaps time, and the improvements
experience will afford, may make it applicable with advantage to
larger vessels.
Having near my kitchen chimney a round hole of eight inches diameter,
through which was a constant steady current of air, increasing or
diminishing only as the fire increased or diminished, I contrived
to place my jack so as to receive that current; and taking off the
flyers, I fixed in their stead on the same pivot a round tin plate of
nearly the same diameter with the hole; and having cut it in radial
lines almost to the centre, so as to have six equal vanes, I gave to
each of them the obliquity of forty-five degrees. They moved round,
without the weight, by the impression only of the current of air,
but too slowly for the purpose of roasting. I suspected that the air
struck by the back of each vane might possibly by its resistance
retard the motion; and to try this, I cut each of them into two, and
I placed the twelve, each having the same obliquity, in a line behind
each other, when I perceived a great augmentation in its velocity,
which encouraged me to divide them once more, and, continuing the
same obliquity, I placed the twenty-four behind each other in a line,
when the force of the wind being the same, and the surface of vane
the same, they moved round with much greater rapidity, and perfectly
answered my purpose.
The second experiment that I propose, is, to take two playing cards
of the same dimensions, and cut one of them transversely into eight
equal pieces; then with a needle string them upon two threads one
near each end, and place them so upon the threads that, when hung up,
they may be one exactly over the other, at a distance equal to their
breadth, each in a horizontal position; and let a small weight, such
as a bird-shot, be hung under them, to make them fall in a straight
line when let loose. Suspend also the whole card by threads from
its four corners, and hang to it an equal weight, so as to draw
it downwards when let fall, its whole breadth pressing against the
air. Let those two bodies be attached, one of them to one end of a
thread a yard long, the other to the other end. Extend a twine under
the ceiling of a room, and put through it at thirty inches distance
two pins bent in the form of fish-hooks. On these two hooks hang
the two bodies, the thread that connects them extending parallel to
the twine, which thread being cut, they must begin to fall at the
same instant. If they take equal time in falling to the floor, it
is a proof that the resistance of the air is in both cases equal.
If the whole card requires a longer time, it shows that the sum of
the resistances to the pieces of the cut card is not equal to the
resistance of the whole one[31].
This principle so far confirmed, I would proceed to make a larger
experiment, with a shallop, which I would rig in this manner. (Plate
VI. Fig. 4.)
A B is a long boom, from which are hoisted seven jibs, a, b, c, d,
e, f, g, each a seventh part of the whole dimensions, and as much
more as will fill the whole space when set in an angle of forty-five
degrees, so that they may lap when going before the wind, and hold
more wind when going large. Thus rigged, when going right before the
wind, the boom should be brought at right angles with the keel, by
means of the sheet ropes C D, and all the sails hauled flat to the
boom.
These positions of boom and sails to be varied as the wind quarters.
But when the wind is on the beam, or when you would turn to
windward, the boom is to be hauled right fore and aft, and the sails
trimmed according as the wind is more or less against your course.
It seems to me, that the management of a shallop so rigged would be
very easy, the sails being run up and down separately, so that more
or less sail may be made at pleasure; and I imagine, that there being
full as much sail exposed to the force of the wind which impels the
vessel in its course, as if the whole were in one piece, and the
resistance of the dead air against the foreside of the sail being
diminished, the advantage of swiftness would be very considerable;
besides that the vessel would lie nearer the wind.
Since we are on the subject of improvements in navigation, permit
me to detain you a little longer with a small relative observation.
Being, in one of my voyages, with ten merchant-ships under convoy
of a frigate at anchor in Torbay, waiting for a wind to go to the
westward; it came fair, but brought in with it a considerable swell.
A signal was given for weighing, and we put to sea all together; but
three of the ships left their anchors, their cables parting just as
the anchors came a-peak. Our cable held, and we got up our anchor;
but the shocks the ship felt before the anchor got loose from the
ground, made me reflect on what might possibly have caused the
breaking of the other cables; and I imagined it might be the short
bending of the cable just without the hause-hole, from a horizontal
to an almost vertical position, and the sudden violent jerk it
receives by the rising of the head of the ship on the swell of a wave
while in that position. For example, suppose a vessel hove up so as
to have her head nearly over her anchor, which still keeps its hold,
perhaps in a tough bottom; if it were calm, the cable still out would
form nearly a perpendicular line, measuring the distance between the
hause-hole and the anchor; but if there is a swell, her head in the
trough of the sea will fall below the level, and when lifted on the
wave will be much above it. In the first case the cable will hang
loose and bend perhaps as in figure 5. In the second case, figure 6,
the cable will be drawn straight with a jerk, must sustain the whole
force of the rising ship, and must either loosen the anchor, resist
the rising force of the ship, or break. But why does it break at the
hause-hole?
Let us suppose it a cable of three inches diameter, and represented
by figure 7. If this cable is to be bent round the corner A, it is
evident that either the part of the triangle contained between the
letters a, b, c, must stretch considerably, and those most that are
nearest the surface; or that the parts between d, e, f, must be
compressed; or both, which most probably happens. In this case the
lower half of the thickness affords no strength against the jerk, it
not being strained, the upper half bears the whole, and the yarns
near the upper surface being first and most strained, break first,
and the next yarns follow; for in this bent situation they cannot
bear the strain all together, and each contributes its strength to
the whole, as they do when the cable is strained in a straight line.
To remedy this, methinks it would be well to have a kind of large
pulley wheel, fixed in the hause-hole, suppose of two feet diameter,
over which the cable might pass; and being there bent gradually to
the round of the wheel, would thereby be more equally strained, and
better able to bear the jerk, which may save the anchor, and by that
means in the course of the voyage may happen to save the ship.
One maritime observation more shall finish this letter. I have been a
reader of news-papers now near seventy years, and I think few years
pass without an account of some vessel met with at sea, with no
living soul on board, and so many feet of water in her hold, which
vessel has nevertheless been saved and brought into port: and when
not met with at sea, such forsaken vessels have often come ashore
on some coast. The crews, who have taken to their boats and thus
abandoned such vessels, are sometimes met with and taken up at sea by
other ships, sometimes reach a coast, and are sometimes never heard
of. Those that give an account of quitting their vessels generally
say, that she sprung a leak, that they pumped for some time, that
the water continued to rise upon them, and that, despairing to save
her, they had quitted her lest they should go down with her. It
seems by the event that this fear was not always well founded, and
I have endeavoured to guess at the reason of the people's too hasty
discouragement.
When a vessel springs a leak near her bottom, the water enters with
all the force given by the weight of the column of water, without,
which force is in proportion to the difference of level between the
water without and that within. It enters therefore with more force
at first and in greater quantity, than it can afterwards when the
water within is higher. The bottom of the vessel too is narrower,
so that the same quantity of water coming into that narrow part,
rises faster than when the space for it to flow in is larger. This
helps to terrify. But as the quantity entering is less and less as
the surfaces without and within become more nearly equal in height,
the pumps that could not keep the water from rising at first, might
afterwards be able to prevent its rising higher, and the people might
have remained on board in safety, without hazarding themselves in an
open boat on the wide ocean.(Fig. 8.)
Besides the greater equality in the height of the two surfaces, there
may sometimes be other causes that retard the farther sinking of a
leaky vessel. The rising water within may arrive at quantities of
light wooden work, empty chests, and particularly empty water-casks,
which if fixed so as not to float themselves may help to sustain her.
Many bodies which compose a ship's cargo may be specifically lighter
than water, all these when out of water are an additional weight to
that of the ship, and she is in proportion pressed deeper into the
water; but as soon as these bodies are immersed, they weigh no longer
on the ship, but on the contrary, if fixed, they help to support her,
in proportion as they are specifically lighter than the water. And
it should be remembered, that the largest body of a ship may be so
balanced in the water, that an ounce less or more of weight may leave
her at the surface or sink her to the bottom. There are also certain
heavy cargoes, that, when the water gets at them, are continually
dissolving, and thereby lightening the vessel, such as salt and
sugar. And as to water-casks mentioned above, since the quantity of
them must be great in ships of war where the number of men consume a
great deal of water every day, if it had been made a constant rule to
bung them up as fast as they were emptied, and to dispose the empty
casks in proper situations, I am persuaded that many ships which
have been sunk in engagements, or have gone down afterwards, might
with the unhappy people have been saved; as well as many of those
which in the last war foundered, and were never heard of. While on
this topic of sinking, one cannot help recollecting the well known
practice of the Chinese, to divide the hold of a great ship into a
number of separate chambers by partitions tight caulked (of which you
gave a model in your boat upon the Seine) so that if a leak should
spring in one of them the others are not affected by it; and though
that chamber should fill to a level with the sea, it would not be
sufficient to sink the vessel. We have not imitated this practice.
Some little disadvantage it might occasion in the stowage is perhaps
one reason, though that I think might be more than compensated by an
abatement in the insurance that would be reasonable, and by a higher
price taken of passengers, who would rather prefer going in such
a vessel. But our seafaring people are brave, despise danger, and
reject such precautions of safety, being cowards only in one sense,
that of _fearing_ to be _thought afraid_.
I promised to finish my letter with the last observation, but the
garrulity of the old man has got hold of me, and as I may never have
another occasion of writing on this subject, I think I may as well
now, once for all, empty my nautical budget, and give you all the
thoughts that have in my various long voyages occurred to me relating
to navigation. I am sure that in you they will meet with a candid
judge, who will excuse my mistakes on account of my good intention.
There are six accidents that may occasion the loss of ships at
sea. We have considered one of them, that of foundering by a
leak. The other five are, 1. Oversetting by sudden flaws of wind,
or by carrying sail beyond the bearing. 2. Fire by accident or
carelessness. 3. A heavy stroke of lightning, making a breach in the
ship, or firing the powder. 4. Meeting and shocking with other ships
in the night. 5. Meeting in the night with islands of ice.
To that of oversetting, privateers in their first cruize have, as
far as has fallen within my knowledge or information, been more
subject than any other kind of vessels. The double desire of being
able to overtake a weaker flying enemy, or to escape when pursued
by a stronger, has induced the owners to overmast their cruizers,
and to spread too much canvass; and the great number of men, many
of them not seamen, who being upon deck when a ship heels suddenly
are huddled down to leeward, and increase by their weight the effect
of the wind. This therefore should be more attended to and guarded
against, especially as the advantage of lofty masts is problematical.
For the upper sails have greater power to lay a vessel more on her
side, which is not the most advantageous position for going swiftly
through the water. And hence it is that vessels, which have lost
their lofty masts, and been able to make little more sail afterwards
than permitted the ship to sail upon an even keel, have made so much
way, even under jury masts, as to surprize the mariners themselves.
But there is besides, something in the modern form of our ships
that seems as if calculated expressly to allow their oversetting
more easily. The sides of a ship, instead of spreading out as they
formerly did in the upper works, are of late years turned in, so as
to make the body nearly round, and more resembling a cask. I do not
know what the advantages of this construction are, except that such
ships are not easily boarded. To me it seems a contrivance to have
less room in a ship at nearly the same expense. For it is evident
that the same timber and plank consumed in raising the sides from a
to b, and from d to c, would have raised them from a to e, and from
d to f, fig. 9. In this form all the spaces between e, a, b, and c,
d, f, would have been gained, the deck would have been larger, the
men would have had more room to act, and not have stood so thick in
the way of the enemy's shot; and the vessel, the more she was laid
down on her side, the more bearing she would meet with, and more
effectual to support her, as being farther from the centre. Whereas
in the present form, her ballast makes the chief part of her bearing,
without which she would turn in the sea almost as easily as a barrel.
More ballast by this means becomes necessary, and that sinking a
vessel deeper in the water occasions more resistance to her going
through it. The Bermudian sloops still keep with advantage to the old
spreading form. The islanders in the great Pacific ocean, though they
have no large ships, are the most expert boat-sailors in the world,
navigating that sea safely with their proas, which they prevent
oversetting by various means. Their sailing proas for this purpose
have outriggers generally to windward, above the water, on which, one
or more men are placed, to move occasionally further from or nearer
to the vessel as the wind freshens or slackens. But some have their
outriggers to leeward, which, resting on the water, support the boat
so as to keep her upright when pressed down by the wind. Their boats
moved by oars or rather by paddles are, for long voyages, fixed two
together by cross bars of wood that keep them at some distance from
each other, and so render their oversetting next to impossible.
How far this may be practicable in larger vessels, we have not yet
sufficient experience. I know of but one trial made in Europe, which
was about one hundred years since, by Sir William Petty. He built a
double vessel, to serve as a packet boat between England and Ireland.
Her model still exists in the museum of the Royal Society, where
I have seen it. By the accounts we have of her, she answered well
the purpose of her construction, making several voyages; and though
wrecked at last by a storm, the misfortune did not appear owing to
her particular construction, since many other vessels of the common
form were wrecked at the same time. The advantage of such a vessel
is, that she needs no ballast, therefore swims either lighter or will
carry more goods; and that passengers are not so much incommoded by
her rolling: to which may be added, that if she is to defend herself
by her cannon, they will probably have more effect, being kept more
generally in a horizontal position, than those in common vessels. I
think, however, that it would be an improvement of that model, to
make the sides which are opposed to each other perfectly parallel,
though the other sides are formed as in common thus, figure 10.
The building of a double ship would indeed be more expensive in
proportion to her burthen; and that perhaps is sufficient to
discourage the method.
The accident of fire is generally well guarded against by the prudent
captain's strict orders against smoking between decks, or carrying a
candle there out of a lanthorn. But there is one dangerous practice
which frequent terrible accidents have not yet been sufficient to
abolish; that of carrying store-spirits to sea in casks. Two large
ships, the Seraphis and the Duke of Athol, one an East-Indiaman, the
other a frigate, have been burnt within these two last years, and
many lives miserably destroyed, by drawing spirits out of a cask near
a candle. It is high time to make it a general rule, that all the
ship's store of spirits should be carried in bottles.
The misfortune by a stroke of lightning I have in my former writings
endeavoured to show a method of guarding against, by a chain and
pointed rod, extending, when run up, from above the top of the mast
to the sea. These instruments are now made and sold at a reasonable
price by Nairne & Co. in London, and there are several instances of
success attending the use of them. They are kept in a box, and may be
ran up and fixed in about five minutes, on the apparent approach of a
thunder gust.
Of the meeting and shocking with other ships in the night, I have
known two instances in voyages between London and America. In one
both ships arrived though much damaged, each reporting their belief
that the other must have gone to the bottom. In the other, only one
got to port; the other was never afterwards heard of. These instances
happened many years ago, when the commerce between Europe and America
was not a tenth part of what it is at present, ships of course
thinner scattered, and the chance of meeting proportionably less.
It has long been the practice to keep a _look-out before_ in the
channel, but at sea it has been neglected. If it is not at present
thought worth while to take that precaution, it will in time become
of more consequence; since the number of ships at sea is continually
augmenting. A drum frequently beat, or a bell rung in a dark night,
might help to prevent such accidents.
Islands of ice are frequently seen off the banks of Newfoundland,
by ships going between North-America and Europe. In the day time
they are easily avoided, unless in a very thick fog. I remember two
instances of ship's running against them in the night. The first lost
her bowsprit, but received little other damage. The other struck
where the warmth of the sea had wasted the ice next to it, and a part
hung over above. This perhaps saved her, for she was under great way;
but the upper part of the cliff taking her foretopmast, broke the
shock, though it carried away the mast. She disengaged herself with
some difficulty, and got safe into port; but the accident shows the
possibility of other ships being wrecked and sunk by striking those
vast masses of ice, of which I have seen one that we judged to be
seventy feet high above the water, consequently eight times as much
under water; and it is another reason for keeping a good _look-out
before_, though far from any coast that may threaten danger.
It is remarkable, that the people we consider as savages have
improved the art of sailing and rowing-boats in several points beyond
what we can pretend to. We have no sailing-boats equal to the flying
proas of of the South Seas, no rowing or paddling-boat equal to that
of the Greenlanders for swiftness and safety. The birch canoes of the
North-American Indians have also some advantageous properties. They
are so light that two men may carry one of them over land, which is
capable of carrying a dozen upon the water; and in heeling they are
not so subject to take in water as our boats, the sides of which are
lowest in the middle where it is most likely to enter, this being
highest in that part, as in figure 11.
The Chinese are an enlightened people, the most antiently civilized
of any existing, and their arts are antient, a presumption in their
favour: their method of rowing their boats differs from ours, the
oars being worked either two a-stern as we scull, or on the sides
with the same kind of motion, being hung parallel to the keel on a
rail and always acting in the water, not perpendicular to the side as
ours are, nor lifted out at every stroke, which is a loss of time,
and the boat in the interval loses motion. They see our manner, and
we theirs, but neither are disposed to learn of or copy the other.