Evening star., 1936-07-26
JyIy 26. 1886

The surprising thing that results from these
delicate weighings by the Pittsburgh scien- !
fists is not that the earth's crust should yield
to the moons pull. but that it should yield
as much as it does. Biologists and geophys-
icists have long supposed that the earth was
much too rigid to yield anything like this
amount.
A number of years ago two Chicago scien-
fists. Nicholson and Gale, devised an ex-
pertinent whereby they hoped to measure
tides in the earth. They buried two 5OO-foot
pipes horizontally. deep below the ground
level, on the premises of the Yerkes Observa-
tory near Lake Geneva in Wisconsin. One of
these pipes was laid east and west and the
other north and south. The pipes were half
filled with water and provided with glass win
dows at either end. An ingenious device was
oontrived to record any change in the level
of the water at the two ends of the pipe if
its level altered as much as one one -hundredth
of an inch.
and set over their pipe lines. it would produce
miniature tides in the water in the pipe. The.
level of the water in the east end of the pipe
would first feel the tide in this miniature
ocean, When the moon had passed overhead
and was descending toward the western h0ri-
zon, the water at the west end of the pipe
would be higher Knowing the attraction of
the moon. it was possible to calculate theo-
retically just how big tide would be raised
in this 500 foot body of water.
The results of these measurements showed
that the water never rose as high as the
theoretical calculations demanded This meant
that the earths crust itself must be yielding
a little bit to the pull of the moon. The differ
ence, then, between the actual observed
height and the height which the water would
have risen if the earth had not yielded showed
just how much the earth's crust tilted as the
moon went by.
From a long series of these very precise
measurements they concluded that the earth
yielded about as much as it would if it we're
made of solid steel. This seemed to confirm
previous ideas which had been held by scien-
fists.
The observations at Pittsburgh, however.
indicate a much larger yield than was indi
Gated by the results in Wisconsin. However.
Pittsburgh scientists went at the problem in
different way They were measuring not the
tilt of the ground with respect to water in a
pipe, but the actual vertical lift of the ground
toward the moon when it was overhead. If
the water pipes in Wisconsin were lifted verti-
cally upward but still maintained their strictly
horizontal position, there would have been no
observed change in the water level of the
pipe at all. How to reconcile the large vertical
lift as determined from the Pittsburgh results
with the tits observed in the pipe experiment
carried on in Wisconsin is still battling
problem.
There have been other experiments that
also bear on this question. For the last ten
years the author has been analyzing thou
sands of observations of the stars which are
used for determining the exact latitude, or
the distance north of the earth's equator, at
which an observatory is situated. The posi-
ton of an observatory can be very accurately
determined by observing the stars, just as the
sea captain determines the position of his ship
by observations of the sun and stars with his
sextant.
However, one can make observations with
much greater accuracy on land than on board
a ship. In fact with very delicate instru-
ment and long series Of observations of the
stars. one could tell that the north end of
10-foot flower bed was farther away from the