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

S E A M A- N S H I P.
■when plying to windward, are the flay fails. We be¬
lieve that it is impoflible to fay, with any thing ap¬
proaching to precifion, what is the petition _of the gene¬
ral furface of a ftayfail, or to calculate the inteniity and
direction of the general impulfe ; and we affirm with
confidence that no man can pronounce on thefe points
with any exa£tnefs. If we can guefs within a third or
a fourth part of the truth, it is all v^e can pretend to 5
and after all, it is but a guefs. Add to this, the fails
coming in the way of each other, and either becalming
them or fending the wind upon them in a dirc&ion
widely different from that of its free motion. Ail thefe
points avc think beyond our poAver of calculation, and
therefore that it is in vain to give the feaman mathema¬
tical rules, or even tables of adjuftment ready calcula¬
ted 5 fince he can neither produce that medium pofition
of his fails that is required, nor tell what is the pofition
33
The theory
itfelf erro¬
neous,
Avhich he employs.
This is one of the principal reafons why fo little ad¬
vantage has been derived from the very ingenious and
promifing difquifitions of Bouguer and other mathemati¬
cians, and has made us omit the aflual folution of the
chief problems, contenting ourfelves with pointing out
the procefs to fuch readers as have a reliffi for thefe ana¬
lytical operations.
But there is another principal reafon for the fmall
progrefs Avhich has been made in the theory of feaman-
(hip ; This is the error of the theory itfclf, Avhich fup-
pofes the impulfions of a fluid to be in the duplicate ra¬
tio of the fine of incidence. The mod careful compari-
fon which has been made between the refults of this
theory and matter of faft is to be feen in the experi¬
ments made by the members of the Royal Academy of
Sciences at Paris, mentioned in the article RESISTANCE
of Fluids. We fubjoin another abftrafl of them in the
following table j ay here col. ill gives the angle of inci¬
dence *, col. 2d gives the impulfions really obferved •,
col. 3d the impulfes, had they folloAved the duplicate
ratio of the fines-, and col. 4th the impulfes, if they were
in the Ample ratio of the lines.
Angle
of
Incid
90
84
78
72
66
60
54
48
42
36
3°
24
18
12
6
Impul-
lion
obferved-
989
958
908
845
771
693
615
543
480
440
424
414
406
400
Impulfe,
as
Sine J.
IOOO
989
957
9°5
835
75°
655
552
448
346
250
165
96
43
11
Impulfe
as
Sine.
ICOO
995
978
95'
9I4
866
8-9
743
669
587
coo
407
3°9
208
i°5
Here Ave fee an enormous difference in the great ob¬
liquities. When the angle of incidence is only fix de¬
grees, the obferved impulfe is forty times greater than
the theoretical impulfe j at *2° it is ten times greater 5
at 18° it is more than four times greater ; and at 240 it
is almoft three times greater. 39
No Avonder then that the deduftions from this theory- ami t'16 <lC-
are fo ufelefs and fo unlike Avhat Ave familiarly obferve.
We look notice of this Avhen Ave were confidering the |e^s<
leeway of a reftangular box, and thus faiv a reafon for
admitting an incomparably fmaller leeivay than what
avouM relult from the laborious computations neceffary
by the theory. This error in theory has as great an in¬
fluence on the impulfions of air Avhen a£Hng obliquely
on a fail; and the experiments of Mr Robins and of
the Chevalier Bordaon the oblique impulfions of air are
perfedly conformable (as far as they go) to thofe of
the academicians on Avater. The oblique impulfions of
the ivind are therefore much more efficacious for pref-
fing the fliip in the diredlion of her courfe than the
theory alloAvs us to fuppofe ; and the progrefs of a fliip
plying to windwrard is much greater, both becaufe the
oblique impulfes of the Avind are more effective, and be¬
caufe the leeway is much fmaller, than Ave fuppofe.
Were not this the cafe, it Avould be impoffible for a
fquare-rigged ffiip to get to windAvard. The impulfe
on her fails Avhen clofe hauled would be fo trifling that
ffie Avould not have a third part of the velocity which
Ave fee her acquire : and this trifling velocity would be
Avafled in leeway; for avc have feen that the diminution
of the oblique impulfes of the Avater is accompanied by
an increafe of leeivay. But Ave fee that in the great ob¬
liquities the impulfions continue to be very confiderable,
and that even an incidence of fix degrees gives an impulfe
as great as the theory allows to an incidence of 40.
We may therefore, on all occafions, keep the yards
more fquare; and the lofs Avhich we fuflain by the dimi¬
nution of the very oblique impulfe Avill be more than
compenfated by its more favourable direction Avith re-
fpeft to the fhip’s keel. Let us take an example of
this. Suppofe the Avind about two points before the
beam, making an angle of 68° with the keel. I he
theory affigns 430 for the inclination of the wind to
the fail, and 1 50 for the trim of the fail. The perpen¬
dicular impulfe being fuppofed IOCO, the theoretical im¬
pulfe for 450 is 465. This reduced in the proportion of
radius to the fine of 250, gives the impulfe in the direc¬
tion of the courfe only 197.
But if Ave eafe off the lee-braces till the yard makes an
angle of 50° Avith the keel, and allows the AA'ind an inci¬
dence of no more than 180, Ave have the experimented
impulfe 414, which, when reduced in the proportion of
radius to the fine of 50°, gives an effedlive impulfe 317.
In like manner, the trim 56°, Avith the incidence 1 2°,
gives an effeftive impulfe 337 ; and the trim 62°, Avith
the. incidence only 6°, gives 3 53.
Hence it would at firft fight appear that the angle
DCB of 62° and WCD of 6° would be better for hold¬
ing a courfe Avithin 6 points of the Avind than any
more oblique pofition of the fails ; but it Avill only give
a greater initial impulfe. As the {hip accelerates, the
Avind apparently comes ahead, and Ave muft continue to
brace up as the ffiip frefliens her way. It is not unufual
for her to acquire half or two thirds of the velocity of
the Avind ; in which cafe the Avind comes apparently
ahead more than tAvo points, Avhen the yards muft
be braced up to 350, and this alioAVs an impulfe no
greater than about 7V. Now this is very frequently
obferved.