The New York herald. [volume] (New York [N.Y.]) 1840-1920, 1853-08-25


is rarely undertaken than such q measurem cK be-
coIoes, when great aocwasyis to be anainods n
We did not care FOR errors OF yard or two in tase
OF several miles, its measurement would be easy
enOughy but when x'tsneds Of an zach 10 seven
miles becomes the limit Terror supposable the mat
ter liz one OF tsanscendAs skull and the executioa OF
such measurcnentis ore Of the most refined opera
tons ever conducted. First OF au the measuring bar
must be 80 accurately compared with and adjusted to
the legal standards Of linear measure as TO eliminate
oil possible discrepancies between The tWo. Without
great care and fidelity in this fundamental step.
also IN the comparison OF different national standards
OF length the various independent surveys executed
wonld give discordant geodetic elements, add the
discrepancies arieing from imperfect comparisons
would % peculiarly difficult Of detection and ccrrec.
uotL But the element most OF all demanding skiu
and care springd from the ssity Of eliminating
the effects Of varying temperatures and moisture,
and OF all the mechanical accidents incident 70 the
use OF the base apparatus So TO construct this ap
paiatus as that the distance between the measuring
points shall, under all circumstances Of varying
temperatures, be absolutely inyarisble, is the
problem which is solved with preemineut ac-
ouracy In Professor Baches base apparatus. The
compensating principle, first used By Colonel
the Massachusetts surveys. was perfected by
correction originally applied by Professor Bache.
Two bars OF different metals (ir0n and brass having
unequal expansive rates are Joined transversey at
the ends by movable levers which bear the contact
points; the attachments being 40 adjusted that the
contact points will not be moved by the expansion
or contraction of this bars due to heat variations
But II the two bars do not respond with the same
facility to variations Oftemperature, the c0mpensa
tion will be incomplete during the progress OF tenn
perature changes Professor Bache by means of
Saxtons pyromeier, "a beautiful arrangement, also
exhibited at the Crystal Palace which consists Of
plain mirror mounted so as to rotate the expert
mental bar varies and thus to reflect the graduations
OF distant arc, into telescope fixed this arc
made numerous experiments on the specific heats Of
iron and brass bars and determined their relative
rtQna that tho ufue
vary their temperatures. through given amount in
equs times He also experimentalIy determined
sarrishes for the two which give taem equal rates
OF absorption. The contacts between the ends of
successive base bars is made in measuring by abut
ting al agate knife edge against agate plane and
contact level INDICATES the standard contact the
tube being worked ba EDWARDS and forwards on the
tTestie, by delicate screw movement, until the
level indicates an exact contact adjustment II the
tnhc be inclined in ascending descending. level
on the radius Of a small graduated arc gives the
angle OF inclination of the bars, and versed sine
correction for this reduces its length to 3 hori-
zontaI equivalent. The tin tube which encloses
the bars, and the iron stifoBing frame work
which supports the bars on rollers, is intended
to Protect the whole from injury, and from the
direct action Of sunshine and weather, thus
somewhat limiting temperature variations. The
whole IL secure against changes Of Texture suf
fcieEt to induce appreciable errors and the comb
nation embraced in tube is as Light and portable
(four men carrying tube in measuring bssesl as it
could safely he made with bars six metres or about
twenty feet in length. The construction Of the
trestles IN Professor Baoas apparatus, 10 quite pc
cuar, and is thought to give unequalled convenience
to the use Of the apparatus. Two tubes and six.
sometimes eight trestles, employed in measuring
a base. The Coast Survey bases are measured with
combined accuracy and rapidty quite uns quailed
in the performances If base measuring apparatus 80
far as We have published means Of comparison, aud
we think Professor Baches apparatus is entitled to
the first rank, in respect to its compensation and
facility Of field use In measuring Dauphin Island
base, four tenths Of mile was the syerage and 1873
tubes, or near seven tenths OF mile, the great
est days work As the Coast Survey will ultimately
require at least eleven base lines for each sec
ton this facility of use becomes M clement &c even
great economical importance in our OWE case
When the base is duly measured no farther direct
measueme OF lines in The section is necessary, but
all the remaining obseVatioas IN the triangulatoD
consist OF angle mcasurcme . on graduated circular
limbs. The facility with hiCh angles can be OBJ
served or measured, with numerous repetitions, as
well as the convenience o. angle instruments for
transportation, make il matter Of primary impor-
lance thus 10 limit recourse to linear measurement
to tho one fundamental base line by reducing all the
subsequent operations to angle measurements and
computations.
Having completed the base line measurcms ut,
each extremity is occupied as primary station. and
observations made to determine its latitude and Ion
gitude, and the aaimuth Of the base line. In the
Coast Survey, attitudes are now habtualIy observed,
(after careful comparisons with the other known
methods and instruments) by the method due
TO Captain A, Talcott. late of the United States
Corps of Engineers. Pairs Of stars are chosen, bav-
ing nearly equal but contrary zenith distances that
by turning the instrument IS0 deg. around vertical
axis, the North and South stars will both the
ing the clamping and their difference Of zenith dis
tancc is automatically measured by wires in the
instrumental field. Hence. by the aid Of star ca
talogues, the latitude is deJuccd very fne spec.
survey ATTITUDES is now set up in the Crystal Palace
Just by the door Of the Crystal Palace Supernten-
dents Office. 1t was made by Windeman, in Wash
ngton. Two or three hundred latitude observations
are usually made at each astronomical primary
station, nearly fifty of which have already been
occupied. The observations for longitude dferen-
ces are made with great care ard labor, by the Sc
yerl methods Of transporting chronometers Of moon
culminations, eciipses Occutatons and the magnetic
telegraph The last method belongs to this country
originated and grew up in the Coast Survey under
the lamented Professor S. G. Walker, and is, de-
cidcdly the best of all where its application is prac
Crystal Palace IS one 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 in an astrono
mical determination of Ongitudes The azimuth or
angle with the north line, made by the base, is &C.
termined with thirty inch theodOiite. Other sta
tiOns next chosen about ten miles distant and
signals are established or heliotropcs 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
in turn similarly occupied and like observations made
On the base extrcmities and other new stations.
Thus results series of triang'es in which ali the
angles are observed, and In one of which the length
OF side is known. From these elements all the re-
naming sides or distances between stations IRE ae
curatcly derived by computations The length of
sides is usually increased in proceeding from the
base some lines Of over ninety miles long having been
observed in New England. This net work of tranglea
makes MP what is called the primary triangiation,
and II is from its discussion that the length Of me-
rdian and latitude arcs the value of the earths "a
dius, its ellipticty and deyiations from regular
ehpsoid with ether grand geodetic results, ore de-
rived. Quito numerous primary stations, (arcady
about ity) occupied for astronomical, latitudc,
and longitude obsefy ations; o step made Necessary
by the marked irregularities Of the earths crust, from
which result quite spprecable differences between the
astronomical ATTITUDES and longitudcs ana the eeode
tic or those derived by computation from tue phi
mary triangulation. AZ the work progresses. con
nectlons viii be made between the primary triaugu
lstlons Of all the nire Atlantic and Gulf sections
and the bases will thus verify each other 80 the
length Of the connecting sides will be derived from
two bases which should give identical lenath the