Evening star., 1898-12-17

writer visited the survey building yesterday
| the very bars used in this final work were
| taken from their storage cases and
| shown to him. There are two of these. a
| pair being all that is necessary. since one
always remains stationary. while the other
! is carried forward and laid end to end with
it. Each bar in itself is essentially two
} bars, one of brass and the other of steel,
so made that they are precisely the same
| in length. at a temperature of p degrees G..
| They are then five meters long, and are
| placed exactly parallel inside a cylindrical
tuhlng which forms an air space whose
temperature may always be read from
thermometers attached. This tubing re-
wolves inside another the latter having an
exterior covering of canvas. with cotton
underneath for insulation against changes
OF temperature. Each bar can be moved in
or out short distance at each end of .he
| tube by turning a brass screw. Each ex-
tremity of the brass rod and each extrem-
ity of the steel one is capped with a glid-
| ing thimble. arranged that when the
brass of one rod is brought in contact with
the brass of another, and likewise steel
with steel the rods themselves are not
Jared. but simply this thimble. grad
uated indicator on each thimble showing
exactly how far it has been moved by ONLY
| the slightest appreciahle pressure. At the
| end of each thimble is a cap of agate with
which the actual contact is made S,1pnos-
ing one OF these Instruments to be set and
the other to be carried forward and placed
heIore 1t, the agate buffers come together
ana the th1nbles move a trtfe, but the
brass and steel rods in the set device re-
main unmoved. One brass rod ALWAYS
comes in contact with the other and the
same IS done wit the steel. The indicators
on the TOUR thimbles show how much space
must ne allowed for pressure of contact.
SPIRIT levels underneath the canvas castings
Indicate when both instruments are level
| Ieiescopic levels on top show their inclines
gong up or down hill. and rectify their
dllgnment. The thermometers state tho
temperature of the air space surrounding
the bars of the instrument lust set down
The instrument is then set and the rear ono
Drought forward. An elongated tent upon
runners Is gradually drawn forward by
team. so as to always screen the heat of
me sun from the men and especially from
the instruments. The latter rest a1wavc
upon tripods Thus we have the crudast
method of measurement-the end-to-end
lletlod-carrled to the highest degree of
accuracy. The greatest possible error al
lowed is sixteenth of an inch TO the mile
Naturally this Is slow work over a hasn
llne of eleven miles. Formerly a single in
terror rod of steel was used, but later the
bfaSS was added, as a measurement in two
metals Is now regarded as much more ac.
curate than that in but one The instru-
ment maker of the survey SAYS he has his
eye upon Il new alloy of nickel and steel
telhg eNperimented with in France and
WhIch. 1t is claimed does not expand or
contract under changes of tenperatura
ne Twin instruments which the writer saw
ana described were invented and manufac-
tured at the survey building and cost
about $l,JJo for the two.
Over Great Distances.
The survey regards its late Colorado work
is inc most interesting chapter of the whole
series IN question. Necessity arising from,
the mountainous nature of the country he
came the mother of some unique methods
u measurements At the same time phe-
nomchaily great distances could be covered
because of the unusual clearness oF ~
lf1y-.,t
able to sight from peak to peak without re1
gard Tor curvature of the earth which pre-
vents long sighting in level regions These
mountain measurements were made by in
Sfuh-ents from hundred to a hundred and
el2hly-three miles apart. What is known
as 4 hel1otrope' was used for flashing sig
hals from one station to the telescope of
another The heliOtrope is similar TO the
Monograph. but has no flashing shutter II
throws steady beam of light At such
great distances the light of the helotrope
appeared through the telescope like a small
star often apparently isolated in space the
supporting mountain summit not being vis-
lDlc. During the autumn lust passed this
star OF light could be distinctly seen over
a distance of $50 miles, though the space
intervening was clouded with the smoke. of
forest fires and though the intervening
mountains We're invisible. The highest of
these mountain stations was Cncompahgrc.
Iul.s If,'IOo 1eet above sea level.
Across the Prairies.
triangulation for the great arc was en-
countered in Illinois and Indiana, where
the country is 50 fiat that the curvature Of
the earth prevents long-distance signaling.
To overcome this giant tripods sometimes
l'N feet high were carried about and erect
ed at each station. These supported the
instruments, but the surveyors stood upon
inclostnS frames, Ike Windmill towers
punt 2bout but not touching the giant tri-
pods, Three hundred and eight stations of
all kinds We're established throughout the
entire tranScontinental work. each of which
Is uIicated by a permanent mark. In the
mountains a copper bolt loaded into the
soma rock was left, While in the plains or
elsewhere when possible surface stone is
always accompanied by another buried be-
meath 1t. All such stones have copper bolts
loaded into them. and sometimes so wedgcd
that they secure themselves more and more
ISHtIF as an effort Is made to extract them.
There are some people mean enough to dig
for these stones in order to get the copper
but nature has efaCed more of the SURVEYS
marks than has humanity. The original
stones laid near the coast of Cane Nav
have already been devoured by the waves
As soon as the leveling accompanying this
work Is entirely completed we will have
some tlgures as TO the relative heights of
the Atlantic and Pacific oceans Some claim
that the Pacific will be found higher than
the other-in fact. this is Il question much
mooted by scientific men. If a difference in
the heights is found the survey can
tell with reasonable accuracy whether
the discrepancy is due to errors in leyelinc
or whether too great to be explained in thar
way. Leveling done by the survey across
Florida some time ago and repeated three
different times indicated that the Gulf of
Mexico was foot higher than the Atlantic
and II Is still an open question whether this
Is really true at these points or whether
the same error was repeated thrice by the
carefully manipulated instruments.