The Washington herald., 1915-08-28

out a years tides for a port in about
| thirteen hours with this machine.
Topography is one of the important
operations in charting the coasts of a
country for it is essential for the navi-
gator to know the character and aspect
of the shores touching the waters that
ne is traversing; their prominent and
characteristic features ale his guide in
navigating his vessel. For this reason
the topographic operations have mostly
been restricted to a narrow margin rare
ly more than three or four miles wide,
along the coast, and surrounding harbors,
bays, and rivers to the head of tide
water.
Several methods of making a tooo-
graphic survey are employed, but the
one almost entirely used by the Survey
Is with the instrument known as the
plane table.
The plane table, for topographic pu--
poses. is universal instrument, and
with it all Of the operations necessary
for producing a map can be performed
with ii in the field with the country as 4
model
Magnetic observations.
NavigatiOnal charts are all made with
reference 10 the true meridian The nav
gators guide upon the sea is the mas
neuc compass, which refers to the mag
nctic meridian. The true meridian and
tie magnetic meridian coincide over very
limited portions of the earth, and those
condolences are constantly shifting po-
sltion. The difference between the true
north and magnetic north at any place
is called magnetic declination or varid-
tion OF the compass. This variation of
the compass must be asCertained and put
on all of the navigational charts
All of p1rlipr land
cially in the eastern half of the United
States. were made with the magnetic
needle. and the magnetic bearing of the
lines are, and Will continue to be, of
record as defining the original bounda-
ries of the land. The land surveyor,
in re-establishing the lines of these sur
veys. must not only know what the pres
ent variation is. but he must know what
change it has undergone since the orig
Inal survey was made By compilation
and observation through many Years the !
Coast and Geodetc Survey is able to |
supply this Information. The charts
must also show the annual change IN |
declination over the areas covered by |
them.
True meridian lines for the use of sur
veyors and engineers are established in !!
with the magnetic observa-
tons at many places. over 1000 mag |
netic stations have been made in the |
United States. or one foi about each 40 |
square miles. In addition to the deter
nipation of the magnetic elements At
many widely distributed points and their
frequent redeterminations for secular
;hanSe, special observations are made |
II several base stations with self-record-
nS instruments for the purpose of OBJ
-aIning continuous records of the cease
ess fluctuations of the earth's magnetism.
The results of the magnetic work are
shown on the magnetic charts and other
wise communicated to the public in nu-
nerous publications on the subject
Important Hydrography.
To the navigator the results of the
work are the most i'm
oortant features represented on a
hart. The chief operation in a hydro
graphic survey is making the sound
ngs for the depth of the water These
ire made either with a hand line or
| sounding machine depending upon
he depth of the water. Power boats
md ships are now almost exclusive
y used in running sounding lines
When sounding in deep offshore wa
ers, piano wire is used on the sound
ng machine. In running sounding
ines the position of the sounding
'oat must be known. and the stagei
I the tide when the various sound
ngs are made.
The photographic operations have
xtended as far out from the coast
s is necessary for the interests of
avigation and have included all har-
ors. channels. bays, etc, as far as
he work has gone. Deep sea sound
ngs have been made extensively, es-
ecially in and about the Gulf stream.
photographic surveys with the lead
me. while developing the slopes of
he bottom. do not with certainty re-
cal all of the dangers to navigation |
n regions like the coast of Maine c
nd in Alaska. where there are iso-
lted rocks and lodges rising up from "
he bottom; or in the tropical re- ,
ions fringed with coral reefs and
Oral heads. the work must be sup
lemented by special examinations y
wing TO the extension of ocean COM c
erce and its increasing demands.
uch special examination work is now g
equired. In order to be certain that =
n area is free of all obstruction to !.
specified depth the area is swept i
yer by what is known as wire +
rag. which is essentially a wire, or r
ire cable. Of any desired length Cup O.
10000 feet. so far used) supported a.
intervals by buoys so arranged that \
le submerged wire can be set at1 il
~ required depth. This apparatus Il
towed through the water by two o
three power boats, and il no OBJ fl