Evening star., 1909-05-02

been found running parallel with the
coast. and it is nearly over the top of
these that the cold bands of water flow.
The peculiarities Of other ocean currents
as well as the characteristics OF the gulf
stream. and the temperature and density
of water between surface and bottom are
among the critical studies Of the coast
and geodetic survey The time is said
not to be far distant when the circulation
of the ocean-direction and velocity of
currents-will be determined with preci-
sion for a given hour of the day.
A query as to when this gigantic sur
the official-fn this instance Mr. f. {.
FISCHER chief Of the instrument division
to whom you PROFOUND 1t. In fact this
surveying work Of Uncle Sans is a per-
manent job The entire survey of the
coast accomplished, it is still necessary
TO watch the unceasing changes which
are taking place in the channels leading
10 the harbors, which, for the greater
paft. are barred with shifting sands Thus
1t seems that the labor of hydrograp phic
surveying must be constantly repeated.
or charts will grow worthless and mis-
lead the navigator. Among the notable
instances of the alteration of harbors is
the advance OF Sandy Hook upon IN.
main ship channel into New York bay the
distance of more than a mile and "A quar-
Ter since the commencement of the past
century.
Perhaps the most intricate and trouble
some problem that ever vexed the mind
OF man is that of the tides There is no
subject SO constantly in sailors thoughts
when approaching harbors and river
mouths as the rise and fall OF the water.
The safety Of ship and life depend often
upon proper understanding OF these par
ticulars. The survey has supplied exact
information on points hitherto unknown.
which are Of the utmost momeltt to the
shipping interests Of the world. It has been
able to do this by noting the daily move
ments Of the tides in many places through
long series Of years. Now their occur
renee for each coming year can be pre-
dicted. Even more. their rise and fall at
all the shipping ports OF the country are
furnished In advance with this informa-
ton.
The observations Of the survey are
sometimes seemingly diverse, yet they are
all intimately related. For example much
of its attention has been occupied with
the magnetic laws that govern the COM
pass needle and induce changes in its
direction. Formerly 1t was supposed that
the north end OF the needle pointed di-
rectly to the north pole OF the earth, at
tracted by some powerful agent in the
polar regions. Persistent observations
and study Of the subject disclosed the
fact that this was not true; the real
direction Of the needle. at any point be-
tween the equator and the poles, inclines
to the horizon. and also to the true me-
ridian: or, in other words. the needle
has dip and a variation, or declination
1t is learned. too, that the direction at
any one place Is not constant but changes
not only during each day, but from year
to year. A street a mile long, laid out
in London during the year 1880 in the
direction indicated by the compass at
that time would have had it's northern
terminus six-tenths of a mile too fa-
east, according to the compass direction
OF 1872. Similar conditions hold i'm this
country. and. indeed. at every spot on
the. globe. The sailor has had no re-
source in avoiding errors occasioned by
these variations but his chart. And the
chart for one year is not good for the
next year unless the laws that govern
the changes are sufficiently well under
stood to predict the compass variations as
accurately as the tides are foretod.