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

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S E A M A
had their extreme breadth before the middle point, and
confequently the bows not fo acute as the ftern. 1 his is
fo contrary to what one would expedl, that it; attracted
attention more forcibly •, and, being fomewhat myfteri-
ous, it might prompt to attempts of improvement, by
exceeding in this lingular maxim. We believe that it
has been carried as far as is compatible with other ef-
f>i fential requilites in a Ihip.
Ofimpor- We believe that this is the chief circumftance in
termine die W^at *S ca^e(* t^e tr'm a &ip i an^ were great\y
beft place 6 to be willted that the belt place for the centre of gravi-
for a fhip’s ty could be accurately afcertained. A praftice pre¬
centre of vails, which is the oppofite of what we are now ad-
gravity. vancing. It is ufual to load a Ihip fo that her keel is
not horizontal, but lower abaft. This is found to im¬
prove her fteerage. The reafon of this is obvious. It
increafes the afting furface of the rudder, and allows
the water to come at it with much greater freedom and
regularity 5 and it generally diminilhes the griping of
the Ihip forward, by removing a part of the bows out
of the water. It has not always this effedl ; for the
form of the harping aloft is frequently fuch, that the
tendency to gripe is diminiihed by immerfmg more of
the bow in the water.
But waving thefe circumlfances, and attending only
to the rotatory energy of the rudder, we fee that it is
of advantage to carry the centre of gravity forward.
The fame advantage is gained to the a6bon of the after
fails. But, on the other hand, the adlion of the head¬
fuls is diminiihed by it •, and we may call every fail
a headfail whofe centre of gravity is before the centre of
gravity of the Ihip ; that is, all the fails hoilled on the
bowfprit and foremaft, and the ftayfails hoifted on the
mainmall; for the centre of gravity is feldom far before
the mainmaft.
Suppofe that when the rudder is put into the pofi-
tion AD (fig. 11.), the centre of gravity could be
Ihifted to g, fo as to increafe q G, and that this is done
without increaling the fum of the produffs p r*. It is
obvious that the velocity of converfion will be increafed
in the proportion of ^ G to qg. I Ins is very pollible,
by bringing to that fide of the Ihip parts of her loading
which were fituated at a diftance from G on the other
fide. Nay, we can make this change in fuch a manner
that p r3 lhall even be lefs than it was before, by
taking care that every thing which we Ihift lhall be
nearer to g than it was formerly to G. Suppofe it all
placed in one fpot in, and that m is the quantity of mat¬
ter fo Ihifted, while M is the quantity of matter in the
whole Ihip. It is only neceffary that w ^ G1 lhall be
lefs than the fum of the produffs p r* correfponding to
the matter which has been Ihifted. Now, although the
matter which is ealily moveable is generally very fmall
in comparifon to the whole matter of the Ihip, and there¬
fore can make but a fmall change in the place of the
centre of gravity, it may frequently be brought from
places fo remote that it may occafion a very fenfible di¬
minution of the quantity J'p r*, which expreffes the
whole momentum of inertia.
A practice This explains a praffice of the feamen in fmall wher-
of feamen j-Jpg or Ikiffs, who in putting about are accultomed to
in putting jace themselves to leeward of the mall. They even
about ex- " . . . *1 • 1 .• _
plained.
N S H I P.
boats by the way in which they reft on their two feet,
fometimes leaning all on one foot, and fometimes on the
other. And we have often feen this evolution very fen-
fibly accelerated in a Ihip of war, by the crew running
fuddenly, as the helm is put down, to the lee-bow. And
we have heard it aflerted by very expert feamen, that
after all attempts to wear {hip (after lying-to in a ilorm)
have failed, they have fucceeded by the crew collefting
themfelves near the weather fore-lhrouds the moment
the helm was put down. It muft be agreeable to the
refleffing feaman to fee this praflice fupported by un¬
doubted mechanical principles.
It will appear paradoxical to fay that the evolution rheevolu-
may be accelerated even by an addition of matter to thetion accele-
ftiip ; and though it is only a piece of curiofity, our rated by-
readers may with to be made fenfible of it. Let m be a<!f*ltl0r|a!
the addition, placed in fome point m lying beyond G
from q. Let S be the fpontaneous centre of converfion
before the addition. Let v be the velocity of rotation
round g, that is, the velocity of a point whofe diftance
from <§■ is 1, and let g be the radius vedtor, or diftance of
a particle from g. We have (Rotation, N°22.) v=z
F * <7 P*
. But we know (Rotation, N° 23.)
fp -\-m • mg2
63
G g2. Therefore v =
Let us determine G g
that J'p g =fP r* M •
F • qs
G^-f. til' mg2
and mg and qg.
Let ot G be called z. Then, by the nature of the
centre of gravity, M -f- w : M~G m : g : g m, and
M ^ a , m M1 , T 1M
p m z, and m' g nr zz -=?— sr. In like
* M+m ’ S M + m*
IVa 771^
manner, M • G g2= ~ - zz. Now m M14- M m2
M + m1
zzMm X M 4- m. Therefore M. ‘ G g2-\- m • g m*
TY/T +-yt
Let n be —
M m X(M -\-m) j _ M m J
= * ’ ~ 1V1 -j- in %
M+w*
, then M * Gs -j- n %2. Alfo G g
M4-^
—n z, being rr
2;. Let ^ G be called c: then
M4- «
q g—c-\-n %. Alfo let SG be called e.
We have now for the expreflion of the velocity
F(<r4-W 20
J'p r14- M « iz*
“ = M X
c-\-n %
J'p r*
M4-« z2
But
fpr2
(Rotation, N° 30)^ Therefore, finally,z»-
Had there been no addition of matter
F c-\-n z
M ^ ce-\-n%-
F c
made, we fliould have had 1;=: X —• It remains to
’ M c e
c
{how, that 2; may be fo taken that — may be lefs than
c-\-n z
find that they can aid the quick motions of thefe light ce-\-nz2
Now, if c be to 2; as c e to 2d, that is, if
be