Evening star., 1900-09-29

Yet they performed the feat Of measuring
over single spans with instruments from
loo to lso miles apart. This high mountain
work was done In Colorado and thence
westward across the Rockies and Sierras to
California.
The longest line taken by the surveyors
at single sight of an instrument meas
urd 1871 miles. An observer at the east
era end OF this line viewed through his tel
escope. mounted on one great mountain
peak. the signals flashed by a mirror sta
toned upon another at the western extrem-
ity This mirror apparatus. known as a
heliotrope. was similar to the heliograph,
used in transmitting war signals
1t reflected the rays of the sun. While
the observer at the telescope station in
question stood watching the bright sun
light flash of the heiotrope. 1s3 miles
toward the west. the great orb Of day
would set before him seven minutes previ-
Gus to the time when the distant signal
ceased to be visible; in other words sunset
occurred at the heliotrope's peak seven min
uteS later than at the telescopes peak to
the west Such was the great distance in
tervening. Had a heliotrope at Point Arena
peen erected sufficiently high to transmit
ray or sunlight across the great arc to
cape May, telescope at that Jersey re-
sort might have viewed Old Soils reflected
sparks three hours, fifteen minutes and four
seconds after sunset at the latter point. The
heHotropes s light often appeared to the dis
tant telescope as small star, isolated in
space, the supporting mountain summit
being at the same time made invisible by
'AAA'A" VI pS SIIiokS from forest nreS.
Monster Signal Towers.
Giant signal towers elevated the survey
ofs instruments above the fiat eastern
country where nature had not supplied
mountain peaks for aid In such feats of
measurement as just described.
Ihe highest combination of towers was
erected across Ohio. Indiana and Illinois.
where tall forest growths interfered. Many
or these structures thap
In height One at Green. Indiana held an
instrument 152 feet above the earth's sur
race Each of these lofty edifices appeared
10 De one huge windmill tower erected with
IN anotner Movements of the signaler or
observer, standing upon the top of the
outer structure, did not disturb the target
or telescope mounted upon the platform OF
the inner In windy weather canvas screens
covered the windward sides of the outer
towers, and thus prevented the vibration of
the instruments supports. As further pro-
lection against exceptionally high Winds
the towers were guyed on all sides with
wire cables. In the Kansas syclone heJt
screens OF thin drilling, such as would tear
IN the lace of a fierce wind, were substi-
tuted Tor those of canvas. which latter were
llaDle 10 carry the entire skeleton structure
or becycloned tower to the ground before
giving way. At Still Pond Nd was erected
curious tower 12O feet tall, which sup
ported target to the height of oI5 feet
This target was the highest above ground OF
an those employed. Between such towers
the surveyors signaled over distances yary-
ing between ten and forty miles.
Work Aided by Mirage Phenomena.
We seldom hear of an American mirage.
Yet during this signaling 1t often occurred
that one tower, absolutely invisible from
another throughout the entire course of
the day, would suddenly and most werdly
rise above the distant horizon just at sun
set. thus effecting a much longer line Of
vision than could ordinarily be utilized.
BY actually laying bars Of metal end to
end the surveyors, In the course of this
stupendouS measurement marked off the
base lines essential to their triangulation
work Thus carefully measuring these
base lines and later measuring the angles
from either end to common point they
by applying a simple formula, could esti-
mate how far away that point might be-
The temperature of each bar was carefully
measured by thermometers inside and it's
effect upon the length of the bar was taken
into strict account. A single pair of these
complicated bars cost Uncle Sam 81 800
Stretched across the country they were
not rested upon the ground, but upon trf-
pods carefully leveled. One style of meas
uring bar utilized in this delicate work was
truly wonder 1t was carried in a trough
filled with melting ice and was thus kept
at the zero point. Being always at the
same temperature its length never varied,
Can the imagination be strained to picture
degree of hair-splitting care exceeding
this? JOHN ELFRETH WATKINS. Jr.