Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 19, Scripture-SUG

93
SEAMANSHIP.
be taken equal t® e, the two fractions will be equal.
But if » be lefs than e, that is, if the additional matter
is placed anywhere between S and G, the complex frac-
c
tion will be greater than the fraclion —, and the velo¬
city of rotation will be increafed. There is a particular
dillance which will make it the greateft poffible, name¬
ly, when 55 is made — — c1-^n ce—c), as will
eafily be found by treating the fradtion
c-(-» %
*4
The rota¬
tion per¬
formed
round a
fpontane-
ous axis.
„ /ith
ce-\-n%
2;, confidered as the variable quantity, for a maximum.
In what we have been faying on this fubjedt, we have
confidered the rotation only in as much as it is per¬
formed round the centre of gravity, although in every
moment it is really performed round a fpontaneous axis
lying beyond that centre. This vras done becaufe it af¬
forded an eafy invefligation, and any angular motion
round the centre of gravity is equal to the angular
motion round any other point. Therefore the extent
and the time of the evolution are accurately defined.—
From obferving that the energy of the force F is pro¬
portional to ^G, an inattentive reader will be apt to
conceive the centre of gravity as the centre of motion,
and the rotation as taking place, becaufe the motnenta of
the fails and rudder, on the oppofite fides of the centre
of gravity, do not balance each other. But we muft
always keep in mind that this is not the caufe of the ro¬
tation. The caufe is the want of equilibrium round
the point C (fig. io.), where the adtions of the water
balance each other. During the evolution, which con-
fifts of a rotation combined with a progreffive motion,
this point C is continually fliifting, and the unbalanced
momenta which continue the rotation always refpedt the
momentary fituation of the point C. It is neverthelefs
always true that the energy of a force F is proportional
\cceteris Paribas') to q G, and the rotation is always
made in the fame diredtion as if the point G w7ere really
the centre of converfion. Therefore the mainfail adts
always (when oblique) by pulhing the ftern away from
the wind, although it flrould fometimes adt on a point
of the vertical lever through C, which is a-head of C.
Thefe obfervations on the effedts of the fails and
rudder in producing a converfion, are fufficient for ena¬
bling us to explain any cafe of their adtion which may
occur. We have not confidered the effedts which they
tend to produce by inclining the fhip round a horizon¬
tal axis, viz. the motions of rolling and pitching. See
Rolling and Pitching. To treat this fubjedt pro¬
perly would lead us into the whole dodtrine of the equi¬
librium of floating bodies, and it would rather lead to
maxims of conftrudtion than to maxims of manoeuvre.
M. Bouguer’s Traite du Navire, and Euler’s Scientia
Navalis are excellent performances on this fubjedt, and
we are not here obliged to have recourfe to any errone¬
ous theory.
It is eafy to fee that the lateral preflure both of the
wind on the fails and of the water on the rudder tends
to incline the fhip to one fide. The fails alfo tend to
prefs the fhip’s bows into the water, and, if fhe were
kept from advancing, Avould prefs them down confider-
the fails ba-ably. But by the flop’s motion, and the prominent
lance each form of her bows, the refiftance of the water to the
ether. fore part Qf the flop produces a force which is diredled
<*5
Different
operations
of the wa
ter on the
fhip and
wind on
upwards. The fails alfo have a fmall tendency to raife
the fhip, for they conftitute a furface which in general
feparates from the plumb-line below. This is remark¬
ably the cafe in the ftayfails, particularly the jib and
fore-topmaft ftayfail. And this helps greatly to foften
the plunges of the fhip’s bows into the head feas. The
upward preflure alfo of the water on her bows, which
we juft now mentioned, has a great effedt in oppofing
the immerfion of the bows which the fails produce by
adding on the long levers furnifhed by the malts. M.
Bouguer gives the name otpaint velique to the point V
(fig. 12.) of the malt, where it is cut by the line CV, Fig. 12.
which marks the mean place and diredtion of the whole
impulfe of the water on the bows. And he obferves, that
if the mean diredtiun of all the adtions of the wind
on the fails be made to pafs alfo through this point,
there will be a perfedt equilibrium, and the fhip will
have no tendency to plunge into the water or to rife
out of it j for the whole action of the water on the
bows, in the diredtion CV, is equivalent to, and may
be refolved into the adtion CE, by which the progref¬
five motion is refifted, and the vertical adtion CD, by
which the fhip is raifed above the water. The force
CE muft be oppofed by an equal force VD, exerted by
the wind on the fails, and the force CD is oppofed by
the weight of the fhip. If the mean effort of the fails
paffes above the point V, the fhip’s bow will be preffed
into the water ; and if it pafs below V, her ftern will
be preffed doAvn. But, by the union of thefe forces,
flie Avill rife and fall Avith the fea, keeping always in a
parallel pofition. We apprehend that it is of very little
moment to attend to the fituation of this point. Ex¬
cept Avhen the fhip is right afore the Avind, it is a thou-
fand chances to one that the line CV of mean refiflance
does not pafs through any mafl ; and the fadt is, that
the fhip cannot be in a ftate of uniform motion on any
other condition but the perfedt union of the line of
mean adtion of the fails, and the line of mean adtion of
the refiftance. But its place fhifts by every change of
leeway or of trim ; and it is impoffible to keep thefe
lines in one conftant point of interfedtion for a moment,
on account of the inceffant changes of the furface of the
water on Avhich fhe floats. M. Bouguer’s obfervations
on this point are, however, very ingenious and original. ^
We conclude this differtation, by deferibing fome °f chief evo-
the chief movements or evolutions. What Ave have faid lutions de-
hitherto is intended for the inftrudtion of the artift* by feribed.-
making him fenfible of the mechanical procedure. The
defeription is rather meant for the amufement of the
landfman, enabling him to underfland operations that
are familiar to the feaman. The latter Avill perhaps
fmile at the awkward account given of his bufinefs by
one Avho cannot hand, reef, or fleer.
To tack Ship.
The fhip muft firft he kept full, that is, Avith a
very fenfible angle of incidence on the fails, and by
no means hugging the Avind. For as this evolution is
chiefly performed by the rudder, it is neceffary to give
the fhip a good velocity. When the fhip is obferved
to luff up of herfelf, that moment is to be catched for
beginning the evolution, becaufe fhe Avill by her inhe¬
rent force continue this motion. The helm is then put
down. When the officer calls out Helm’s a-lee, the
fore-fheet, fore-top boAvline, jib, and flag fail fheets for-
Avard