Wessington Springs herald. (Wessington Springs, Aurora County, Dakota [S.D.]) 1883-1891, 1884-09-26
twaty-sourth, or fifteen degrees. IN "A

The distance is therefore fifteen de-
grees. The sun cannot be at his great
est height in two places at one moment.
So noon Cor, indeed, any time of dav)
will come aboard ship at irregular in
tervals, because the vessel moves to-
ward or away from the sun.
Navigators. have, by common con
sent, selected Greenwich Observatory,
London, as the standard time. With
his sextant, when the sun is rising or
falling most rapidly, the Captain takes
an altitude. He notes the Greenwich
time, by his chronometer, at the same
moment. By simple rules he calculates
the time of day from the suns height.
Perhaps he see that the latter is slower
than the former four hours and
forty-eight minutes. Four hours would
be equal to sixty degrees, and in forty
| eight minutes (four-tifths of an hour)
the sur passes over twelve degrees. So
the ship is seventy-two degrees from
Greenwich, or her longitude is seventy
two degrees west, because the ships
time was slow of the chronometer. Lati-
fude is seldom in error, but, as this cal
~culation is dependent on the exact per-
formance of a cock, longitude may vary
from the truth with the unfaithful time
piece. o.p
Measuring the distance between the
moon and another heavenly body is tho
best check, yet discovered, upon this
evil. That space cannot vary, wher-
ever observed. The sun, a star, or
planet may be used, but we must al
ways have the moon, her rapid motion
being the basis of this process. The
altitude of each body is taken simulta-
neously with the lunar distance. There
are several formulas for clearing this
apparent distance, from atmospheric
influence, from the error of the eye in
not being at the carth's centre, and,
chiefly, from the disparity between the
sides of the spherical triangle, and one
of straight lines. But, the true distance
being found, we have only to refer to
the almanac, which tabulates these dis
tances for every three hours of every
day at Greenwich. Comparing our
result, we find from the one approxi-
mating to it the time of day at the OBJ
servatory. The difference between this
and the chronometer time shows the
error of that instrument.
The aim oNthe writer is not to give a
| V4CW W udutlChl SClcuCE.
Otherwise he might explain Summers
method of finding the latitude andlongi-
tude, double attitudes, and the applica-
ton of meridian observations, or the
rules for finding the time to all heavenly
bodies. A ship-master should under
stand all these. AS q fact, few in the
American merchant marine do more
than to work dead reckonings, find lat
tude at noon, and longitude once a day
by the sun, as herein described.
The commanders of English ships are
required to be very attentive to the va
ration of the. compass. Their merchant
men are largely built of iron, and the
magnetic needle has a local deviation
which changes with every course, and
is completely reversed by crossing the
equator. The simplest means of ascer-
tanning the variation is by observation
OF the suns bearing at rising or setting.
A table gives the true bearing. Then
the difference between this and the ap-
parent bearing shows the error of the
compass. Or, instead of reckoning the
suns position from east and west
(%amplitude), we may observe his
bearing from north or south (azimuth),
by means of acompass casting a shadow
on such point, at q time, say, when he
is halfway to meridian. Then, by a
formula we calculate his true bearing.
AS before, a comparison Of the two
gives the variation. But, for ordinary
purposes, the variation Of the magnetic
needle is laid down on the charts with
sufficient accuracy. Yet every master
ought to test his compasses when leav-
inC mort NJ. makinU land
Good charts are most essential in the
closing days of a voyage. This is the
stage of navigation where the error of
popular opinion is most conspicuous.
AS an excuse for disaster, it is often
said either that the master was entering
the port for the first time in his expert
ence, or that he mistook one headland
for another. Let us see if that should
extenuate disaster.
We are nearing land. BY our chart,
or by calculating course and distance,
we find that Point A, a bluff, two hun
dred feet high, lies northeast, distance
one hundred and fifty miles. When
Point A bears north Chat is while pass
ing it), Point B, long and low. should
be seen to the east-north-east, and,
when half way between these two
points, G Island, a small rock, should
appear Just beyond Paint B. The con-
tour and trend Of the coast varies thus
(even as no two human faces are ex-
actly alike), and is defined by careful
surveys.
Our duty is now plain. We must
steer right for point A. But, after a
run of only one hundred miles, we see
land. Our chronomcter is fast. 1t
gave more time between the ship and
observatory than actually exists. Or,
we sail one hundred and fifty miles, and
no land appears. Our ~chronometer is
slow; we have more time to run up than
the difference of the two watches
would indicate. Changing our course
to atone for either one of these errors,
we still stere for the same point. But,
the time being at fault, we see land dis
tinctly only when between Point A and
Point B. We take a bearing of each,
and draw corresponding lines on the
chart. They must cross at the very
spot where the ship now is. Placing
one leg Of the compass on this spot. the
Captain can measure on the chart the
H.