The times., 1901-05-19

BOSTON. May lS.-The total eclipse of
the sun which occurred on the opposite
side of the world early yesterday morning
was an event of such astronomical in
terest that il drew to the islands of the
Indian Ocean scientists from all parts of
the world. While the observation of that
ever-interesting phenomenon, the passage
of the moon between the earth and the
sun, was the chief purpose of this as
Economical pilgrimage, an opportunity
was also presented to the members of the
various expeditions To study in that dis
tant portion of the globe other scientific
problems incidental 10 astronomy, per
haps, but not connected with the eclipse
itself. a notable example of this supple
mental work should be developed in the
geodetic observations undertaken by the
expedition from the Massachusetts In
stitute of Technology. under Mr.. Alfred
+. Burton, professor of typographical en-
gineering. to the Island Of Sumatra
Geodesy, OF course, deals with the shape
and size OF the earth, and its measure
ments are made on the largest scale.
Starting with the work of the practical
surveyor, this new science has within
the last quarter of a century made such
progress that both astronomy and higher
physics are regularly utilized in the solu-
tion Of its problems The accurate meas
urement OF degree of longitude in the
seventeenth century furnished Newton
with the required knowledge Of the earths
mass and consequently the basis of his
whole theory of gravitatiOn. which
modern science rests as upon its first law.
II the earth were perfect sphere the
problem presented to the geOdesist would
be comparatively simple one, but since
Scientists have learned that it is shaped
like a tomato. or, more technically, that
it is an oblate spheroid, the determine
ton OF its exact size and connguration
has become an extremely delicate matter.
In this undertaking there are two sources
of information, one being the measure
ment of the length of the curvature of
the earths surface between two points
whose latitude and longitude are exactly
defined astronomicaIly. and the other the
measurement of the radius Of the earth
at any given point by pendulum snow
ing the force OF gravitation.
The first OF the methods is based upon
the principles OF surveying and is best
illustrated in the recent completion by
the United States Coast and Geooeuic Sur
vey OF the measurement Of an arc OF the
thirts ninth parallel OF latitude from the
Atlantic to the Pacific. This is the great
est undertaking Of the kind ever carried
10 completion. and when combined with
the contemplated measurements of arcs
in other parts Of the world, viii give for
the first time an exact knowledge of the
earth's size and shape. The worl. OF
measuring this arc was begun twenty
six years ago and is SAID to have cost !
about S500.G00. The terminal points are
Cape May. n J.. and Point Arena, Cal,
the route having taken the Government
surveyors over plains. valleys mountains
rivers, and lakes. while carrying their
trlangulations from ocean 10 ocean.
PLANS OF SCIENTIFIC WORK.
IN order TO make an accurate survey
across 50 large an area, a base line of
considerable length must first be meas 1
ured with great exactness. from which.
by measuring of angles, the location OF
other points, such as the tops Of noun
tains and hills, is determined. and tri- /
angles are built on triangles indefinitely /
until it seems expedient to prove the I
work by means Of another base line. The !
first base line may be only few miles in /
length and may run in any direction so 4
long as its exact location on the surface 1
of tie globe Is known. II is usually 4
measured on level ground. but this is not /
necessary, as in any event il must be cal
culated at mean sea Ievel-which in all
geodetic work is taken as the normal
surface of the earth.
The sides of some of the triangles which
are built upon the original base line may
be fifty CT sixty miles in length, requir-
ing the use of very interesting device
called heliotrope or heliograph. A ray
OF light is thrown through two gIobular
sights toward the observer who is able
to define the poiIt accurately with the
telescope of his transit 1t is a remark
wnen he sees for the first time on a noun
tain top fifty miles away the little bead
OF light that marks sharply an important
point IN the triangulatlOn. All the points
in the Government triangulatiOn are indi-
Gated by permanent monuments, but the /
long line across the country. though per
fectiy measured on paper from the tl-
angHlatlOn. need not. Of course, be ac-
tualiy marked except at its two ends
IN the past the usual methods OF meas
uring the base line were cumbersome ana
slow, consisting Of the accurate adjust
ment, end 10 end, Of rods Of wood or met
al The expansion of these rods on ac-
count Of changes Of temperature had 10
be allowed for, and constituted trouble
some element. A great Improvement on
this method has been made through the
work Of the Institute Of Technology,
which. in the course Of its geodetic stud
les, undertook 10 improve the method Of
measurement Of base lines by the steel
tape which can be used more eXpedi-
tiously than rods, but which had been
liable TO greater errors, arising from tenn
perature changes, than the rods them
selves
NOVEL PSYCHOLOGICAL WORK.
The first novelty introduced at the in
stitute WAS the measurement of the pre-
else average temperature of the entire
length OF the tape. usually 100 metres, by /
the use Of the thermophone, an ingenious I
electrical device invented by two institute
graduates which accurately measures the
errors caused by expansion and contrac-
tion. The sag OF the tape between the
various supports was another difficulty
This was finally overcome by means Of
an adjusted weight which stretches the
steel Of the tape to measure 100 metres /
despite the sag These methods were 1
recently tested by the Coast and GeOdetic
Survey in the measurement Of a base line
in Teas with exceedingly satisfactory
results.
1t is natural that special interest should
be taken in geodesy ~ the institute, as
its president. DR. Henry s. PTitchett
was formerly the head Of the United !
States Survey. The connection between |
the Institute and the Survey has always I
indeed, been close. and important work
has been entrusted By the Government 10 |
the Surnatra expedition il relation 10 !
pendulum observations for determining !
the earths ngure. The pendulum work /
Is o! tie most delicate character the |
pendulum itself being completely encased
10 prevent any outside interference and !
fits slightest variations are detected by a
clusc-range telescope. At the equator !
which shows tie greatest distance from | 1
the centre OF the earth, the attraction of 1 ,
erayitation exerts the least force. ana The :
H