s
of
V
6 ,
at
off
e-
uii
ii.
va-
nces
for
m*
vc-
dif-
set
ling'co-ffer*c asth*
NAVIGATION,
29
The Log-Book.
ii*
K
i &
Courses.
Winds.
Observations and Acci-
dents. h-
Day of-----
Fair weather, at four thisafternoon. I took mv depar-ture from the Lizard, in the
1
2
3
7
7
s WbS
North.
latitude of 50°0' north, it bear-ing NNE, distance 5 leagues.
4
7
5
6
7
7
NEE
7
7
i
8
7
t
9
6
i
ssw
10
6
i
\
1112
6
6
swtw
EAS
The gale increasing andbeing under all our sails.
1
2
o
b
6
6
t
i
i
NNE
After three this morning, fre-quent showers with thick wea-
flier till near noon.
4
7
5
8
i
' '
6
S
t
7
8
sw
!
8
9
8
8
i
ENE
The variation I reckon to
—
J—
be one point westerly.
10
9
SW.-W
1 I
j 12
8
8
i
NEAE
Thus lias been shewn bow to determine the ship’s place by■what is called the dead reckoning according to the several me-thods or kinds of sailing. But as the dead reckoning is not altoge-ther to be depended on, it is found necessary to correct the accountby means of celestial observations, by which the latitude and lon-gitude of the place of observation is ascertained. For these andthe methods of finding them, see Astronomy, Geography, andJ .ONGiTUDE. The observations at sea are chiefly made with Had-ley’s quadrant, and because of its extensive use, and being par-ticularly adapted to this purpose, we shall here describe it, ant.point out its application.
or xi adeey’s Quadrant.
The form-of this instrument, according to the present motconstruction, is an octagonal sector of a circle, and thereforetains 45° •, but, because of the double reflection, the limb ivided into 90°. Plate CXXVIl. fig. 1, represents a quadraithe common construction, of which the following are -the pi-inparts •. ABC, the frame of the quadrant; BC, the arch or limb;the index; ab, the subdividing scale; E, the index-glass; Ffore horizon-glass; G, the back horizon-glass; K, the coloursdark-glasses; .HI, the vanes or sights. The frame of the qua<consists of an arch BC, firmly attached to the radii AB, AC, ware bound-together by the braces LM, in order to strengthen itprevent it from warping. The index D is a flat bar of brassturns on the centre of the octant: at the lower end of the ithere is an oblong opening; to one side of this, opening the vcscale is fixed, to subdivide the divisions of the arch ; at the e:the index there is a piece of brass, which bends under thecarrying a spring do make the subdividing scale lie close tdivisions. It is also furnished with a screw, to fix the index iiVOL.-IV, - NO. 15".
The Log-Book.
Courses Correct.
Dist.
Dfft.
Lat.
Dili.
Lou.
w.
SSW
50
46.2
29.4
SAW
19
18.6
5.5
SW
49
29.7
45.5
SWAS
24.5
20.2
'20.0
SW4S
25.5
19.5
24.6
134.2
125.0
Hence the ship, by account, has come to the latitude of 47 c 46*north, and has differed her longitude 2° 5' westerly ; so this day Ihave made my way good S. 31° 31' W. distance 157.4 miles. Atnoon the Lizard bore from me N. 31° 31' E. distance 157.4 miles;and having observed the latitude, I found it agreed with the latitudeby account.
[ desired position. The best instruments have an adjusting screwfitted to the index, tint it may be moved more slowly, andwith greater regularity and accuracy, than by the hand. It is pro-per, however, to observe, that the index must be previously fixedI near its right position by the above-mentioned screw. Upon thel index, ami near its axis of motion, is fixed the index-glass E, aplane speculum, or mirror of glass quicksilvered. It is set in abrass-frame, and is placed so that its face is perpendicular to theplane of the instrument. This mirror being fixed to the index,moves along with it, and has its direction changed bv the motionthereof; and the intention of this glass is to receive the image ofthe sun, or any other object, and reflect it upon-either of the twohorizon-glasses, according to the nature of the observation. Thebrass-frame with the glasses fixed to the index by the screw c: theotherscrew serves to replace it in a perpendicular position, if by anyaccident it has been deranged. On the radius AI> of tire octant aretwo small speculums, called Horizon-Glasses F, G. The surfaceof the upper one is parallel to the index-glass, and that of thelower one perpendicular thereto, when o on the index coin-cides with a on the limb. These mirrors receive the reflectedrays, and transmit them to the observer. The horizon-glasses arenot entirely quicksilvered ; the upper one F is only silvered on. itslower half, or that next the plane of the quadrant, the other halfbeing left transparent, and the back part of thb frame cut away, thatnothing may impede the sigfit, through the unsilvered part of the] glass. The edge of the foil of this glass is nearly parallel to\hej plane of-the instrument, and ought to be very sharp, and without aflaw. The other horizon-glass is silvered at both ends. In themiddle there is a transparent slit, through which the, horizon maybe seen. Each of these glasses is set in a brass-frame, to whichthere is an axis passing through the wood-work, aad it is fitted to a1 I ■ lever