Weekly national intelligencer. [volume] (Washington [D.C.]) 1841-1869, 1853-08-27

this reduces its length to horizontal equivalent Inc
lin tube which encloses the bars and the iron shenin2
framework which supports the bars rollers intend
ed 10 protect the whole from DjuryNnd from the direct
action Of sunshine and weather. thus somewhat limiting
temperature variations The whole IS secure against
changes OF Texture sufficient TO induce appreciate err
rors and the combination embraced in tube is as light
and portable flour carrying tube in measuring
bases as IL could safely be made with bars six metres
or about twenty feet in length The construction Of the
trestes in Professor BAcHEs apparatus is quite peculiar
and Is thought to give unequaed convenience TO the use
OF the apparatus Two tubes and six or sometimes eight
trestes are employed in measuring base The Coas
Survey bases are measured with combined accuracy
and rapidly quite unequaled in the performances Of
base measuring apparatus, far as we have published
means OF comparison, and We think Professor BAcHE
apparatus is entitled TO the first rank in respect to Its
compensation and facility Of field use. In measuring
Dauphin island base four-enths Of mile was the ave-
rage and lS3 tubes seyen-tenths Of mile, was
the greatest days work. As the Coast Survey will ult
maey require eleven base lines one for each section
this facility Of becomes eementof even great eco
nomical importance in our own case
When the base is duly measured, no further direct
measurement Of lines in the section is necessary but all
the remaining observations IN the rianguatlon consist
Of angle on graduated circular limbs The
facility with which angles be observed or measured
with numerous repetons, well as the convenience OF
angle instruments for transportation, make It matter OF
primary importance thus TO limit recourse TO linear mea-
suremen 70 the one fundamental base line, by reducing
all the subsequent operations TO angle measurements ana
computations.
tremty occupied primary station and observations
made to determine its latitude and longitude and the AL
muth Of the base line In the Coast Survey ATTITUDES
ere new habitualy Observed aer careful comparisons
with the othor methods and instru-nts,) by the
method due to Capt A TALcoTr late Of the United States
corps of Engineers Pairs of stars are chosen having
nearly equal but contrary zenith distances SO that by
turning the instrument 1862 around the vertical axis the
North and South stars will both the meridian in the
zenith telescope field without changing the clamping,
and their difference Of zenith distance is micromehcaiy
measured by wires in the instrumental field Hence by
the aid Of star catalogues, the latitude is deduced.
very fine specimen Of the Zenith Telescope as modified
for Coast survey latudes, is set up in the Crystal
Palace just by the door Of the Crystal Palace Supern-
tendents Office 1t was made by WINDEMAN, in Wash
ngton Two or three hundred latitude observations are
usually MADE at each astronomica primary station nearly
fifty of which have already been occupied The observa-
tons for longitude differences are made with great care
and labor by the several methods of transporting chro
nomeers, Of culmnations eclipses, cccultaons,
and the magnetic telegraph. The last method belongs to
this country originated and grew up in the Coast Sur
vey, under the amented Prof 5. c. WALKER and IS de-
cdedy the best Of all where its application is practical
BILE. Among the instruments exhibited at the Crystal
Palace is Of the portable transts which is used for
determining the local time by observing the instant OF
meridian transit for known stars This local time Is, OF
course, requisite astronomical determination OF
longitudes The azimuth angle with the north line,
made by the base is determined with thirtyinch tEeo-
dote Other stations are next chosen, about ten miles
distant and signals established heiotTopes posted
The horizontal angles which these signals make with the
base line and with each other observed with great
care about hundred times each. These stations are II
turn similarly occupied and like observations made on
the base extremites and other stations Thus re-
suits series OF tranges in which all the angles are OBJ
served and in of which the length Of side is known
From these elements al the remaining sides or distances
between stations are accurately derived BY computations
The length Of sides is usually increased in proceeding
from the base lines Of ninety miles long having
been observed in New England This network Of TRAN
giles makes up what is called the primary triangulation,
and is from its discussion that the length of meridian
and latitude arcs the value of the earths radius its
eiptcty, and deyatons from regular elipsoid, with
other grand geodetc results are derived Quite hume
primary stations already about fifty are occupied
for astronomical latitude, and longitude observations
step made necessary by the marked Irregularities Of the
earhs crust from which result quite appreciate differ
between the astronomical ATTITUDES and longudes
and the geodetic, those derived BY computation from
nexions viii be made between the primary trianguatons
" all the nine Atlantic and Gulf sections and the bases
win thus verify each other the lengths of the connect
Ing sides win be derived from bases which should
give identical lengths If the previous operations have been
all correct Besides this the Of the angles in each
triangle after correcting for spherical excess be
equal TO right angles, thus presenting constant
check on the measurements Of horizontal angles. In
order 70 make stations visible great distances in pri
mary observations, the heliotrope heosat is used
This instrument consists Of mirror, go mounted and
connected with train Of coekwork that it be made
TO reflect IN image Of the constantly into the distant
Observers theodoite the minor by its movement exactly
IolIoWiDg the san mouon sucn neloope s EXHIBIT
ed at the Fair
As the primary triangulation progresses secondary
triangulation is begun, basing the primary lines,
though its component lines usually shorter and
tend TO much greater number Of stations, which
occupied with smaller instruments, and smaller number
Of observations at each station made to suffice very
fine l2nch theodolite made by Gambey, in Paris and
exhibited in special showcase at the Fair is used fer
observing attitudes and azmuths in the secondary tr
angulations, where the lines longest 1t is also
poyed IN the primary triaDgulaops Of the southern sec
tons, where the lines Short and where from the fat
and wooded character Of the country it is sometimes
necessary to raise the instruments tripods Of forty feet
height TO command open lines Of vision The result Of
much experience in arranging these tripods has been the
complete separation Of the supports for the instruments
and for the observer Smaller theodolites used in
observing the shorter secondary lines, and specimen
(sizinch) is also exhibited tertiary triangulaon pro-
ceeds from the secondary the lines Of which become quite
short and very numerous to give generally two
more points, accurately determined, within the limits of
each sheet Of soundings topography The topography
is then executed by using the planetable specimen Of
which instrument is exhibition. The topographer,
with his planetable determines graphically by inersect
ing lines the position Of all important points and objects
on sheet Of stretched drawngpaper on which his tri
angnlation points are duly projected to scale Of
l-l00O0th the natural scale. Around the points thus
determined the topographer works in by the eye and
auxiliary instruments the details Of the ground and runs
with the ground surface cut out by series Of horizontal
planes taken twenty feet apart
At this point the photography begins The sounding
party IS furnished with large sheet Of paper on which
careful projection, set Of parallels and merdians is
drawn, and the several points Of triangaation falling
within the contemplated field Of soundngs accurately
plotted Plane-tablc points, and the planetable shore