Saturday press. [volume] (Honolulu, H.I.) 1880-1885, 1884-12-13
Distance Of Objects at Sea

1t IS amusing to note how ignorant many
ordinary scamcn and nearly all sea travellers
are OF such matters as the distance OF the sea
horizon, the way in which ships place at sea
is determined and other such matters-which.
all seamcn nigh, be expected to understand,
and most people Of decen, education might be
expected TO have learned something about at
school. Ask sailor how far off a ship may
be, which is hull down, and he will give You
an opinion based entirely on his knowledge of
the ships probable size, and on the distinct.
ness with which he sees her. This opinion is
often pretty near the truth bu, it may be
preposterously wrong IL his idea Of the ships
real sire is very incorrect, and is sometimes
quite wrong even when he knows her size
somewhat accurately.
ANY notion that the distance may be very
precisely inferred from the relative position Of
the hull and the horizon line seems not to
enter the average sailors head. During my
last journey across the Atlantic we had several
curious illustrations Of this. For instance, on
one occasion steamer was passing at such
distance as to be nearly hull down, From her
character it was known that the portion of her
hull concealed was about twelve feet in hight,
while II was equally well known that the eye
of an observer standing on the saloon passen.
gers deck on the City Of Rome was about
thirty feet above the water level A sailor
asked Thy way of EXPERIMENT. how far off the
steamer was, answered, six or seven miles.
e But she is nearly hull down, some one said
to him. IL didn't say she warnt, as knows
on" was the quaint but stupid reply. Now,
1t might be supposed to be generally known
TACT, that even as seen from the deck of one Of
the ordinary Atlantic steamers, the horizon is
fully six miles away, the hight Of the eye being
about ~ or 20 feet, and that for the concealed
portion Of the other ships hull distance Of
four or five more miles must be allowed; so
that the mans mistake was gross one. And
several other cases Of similar kind occurred
during my seven days journey from Queens.
town to New York.
The rule for determining the distances Of
objects at sea, when the hight of the observers
eye and the high, Of thc'conecaled part of the
remote object above the sea level are both
known. are exceedingly simple, and should be
well known to all. Geometrcally the dip Of
the sea surface is eight inches for mile, four
times this for 1--WE miles, nine times this for
three miles, and so forth; the amount being
obtained by squaring the number Of miles and
taking so many times eight inches. But, in
reality, we are concerned only with the optical
depression, which is somewhat less, because
the line of sight 10 the horizon is slightly
curved THE concavty Of the curve being
turned downwardl. Instead Of eight inches
for mile, the optical depression is about six
inches at sea, where the real horizon can be
observed. But, substituting six inches lor
eight, the rule is as above given.
Six inches being half foot, We obtain the
number Of sis.inch lengths in the hight Of an
observers eye by doubling the number of fee,
IN that hight; the square root Of this number
OF sis'inch lengths gives the number OF miles
in the distance Of the seas horizon. Thus,
suppose the eye Of the observer rs feet above
the sea level; then We double Is, getting 20,
the square roo, Of which is 8; hence the horizon
lies at distance at six miles as seen from an
elevation Of IS feet. For heigh, of 90 feet,
which Is about that OF an observer on the best
deck Of the City Of Rome, We double 90 get
ting 30, the square root of which is ~ hence,
as seen from that deck, the horizon lies at dis
lance Of 7 ITO miles. II the depth of the part
OF distan. ships hull below the horizon is
known, the distance OF thatship beyond the hor
sons obtained in the same way. Thus. suppose
thedepth ofthe part concealed tobe Z, feet, then
We take The square root OF twice -, OF "a,
giving 40, showing tina, the ships distance
beyond the horizon is 9-Io mites. Hence,
ia ship is seen so far hull down. from the hull
Of the City Of Rome, VIC infer that its distance
ss 90 miles beyond the distance OF the hor
i1on. which We have seen to be ? no miles
giving fos that ships distance "a a, miles.
And with like ease may all such cases be dealt
~ith.--Rfshad 4. f,ssfor. ~ Nsunasi.s
Wsskt cA,pnisls.