Sacramento daily record-union. [volume] (Sacramento [Calif.]) 1875-1891, 1884-11-01
Distance of objects at Sea.

1t Is amusing To note now ignorant many
ordinary seamen and nearly all sea travel
ers are Of such matters as the distance of
the sea horizon, the way in which 4 ships
place at sea is determined, and other such
matters-which all seamen might be eX-
pected to understand, and most people Of
decent education might be expected to
have learned something about at school
Ask a sailor how far oil ship may be,
which is hull down, and he will give You
an opinion based entirelyon his knowledge
of the ships probable size, and on the dis
tinctness with which he sees her. This
opinion is often pretty near the truth; but
OF may be preposterously wrong if his idea
if the ships real size 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 illustra-
tions Of this. For instance, on one occasion
a steamer was passing at such Il distance as
to he nearly hull down. From her char
acter it was known that the portion of her
hull concealed was about twelve feet in
hight, while it was equally well known
that the eye of an observer standing on
the saloon passengers deck on the City of
Rome was about thirty feet above the
water level. \ sailor, asked tby way Of
experiment. how far off the steamer
was, answered, "Six or seven miles"
But she is nearly hull down. some
one said to him. " didn't say she warn't,
as knows on" was the quaint but stupid
reply. Now, it might supposed to he
generally known fact, that even as seen
from the deck of one of the ordinary At
lantic steamers the horizon is fully six
miles away, the hight Of the eye being
about IS or 20 feet, and that for the con-
cealed portion Of the other ships hull
distance Of four or five miles more must be
allowed so that the mans mistake was
gross one. And several other cases of a
similar kind occurred during my seven
days journey from Queenstown to New
York
The rule for determining the distances
of objects at sea, when the hight of the
Observers eye and the hight of the con-
cealed part of the remote object above the
sea level are both known, are exceedingly
simple, and should be well known to all
COMMERCIALLY the dip of the sea surface is
eight inches for mile four tines this for
two miles nine times this for three miles,
and so forth; the amount being OBJ
tanned by squaring the number
times eight inches But, in reality, we
are concerned only with the optical de-
preston, which IS somewhat less, because
the line OF sight to the horizon is slightly
curved (the concavity of the curve being
turned downward. Instead Of eight inches
for mile, the optical depression Is about
six inches AZ sea, whore the real horizon
can be observed. Bat, substituting six
inches for eight, the rGle is rs above given
Six inches being half foot, We obtain
the number Of six-inch lengths in the
hight of an Observers eve by doubling the
number Of fee. in that hight; the square
root of this number Of siX-inch lengths
gives the number Of miles in the distance
OF the seas horizon. Thus, suppose the
eve of the observer Is feet above the sea
level; then we double Is, getting SO,
the square root of which is 9; hence the
horizon lies at distance Of six miles rs
seen from an elevation Of Is feet For
hight Of 30 feet, which is about that Of
the eye of an observer on
the best deck of the City of
Rome, we double 50, getting 60, the square
root Of which is II; hence, as seen from
that deck, the horizon lies at Il distance of
40 miles If the depth of the part of
distant ships hull below the horizon is
known, the distance of that ship beyond
the horizon is obtained in the same way
Thus, suppose the depth Of the part con-
cealed to be y feet, then we take the
square root of twice u, or p, giving 40,
showing that that ships distance beyond
the horizon is 4 9-l0 miles. Hence, if
ship is seen so far hull down, from the hull
of the City of Rome, We infer that its
distance is 4 $40 miles beyond the
distance of the horizon, which we
have seen to be 7-l0 miles-giying for
that ships distance l. 3-5 miles. And
with like ease may all such cases he dealt |
with Richard y. Proctor, in Newcastle
Weekly Chronicle.