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HOW MUCH DOES A POUND
WEIGH?
By W. A. DILL
T H A T a "pound's a pound the
world around," is true enough
for all ordinary business
transactions, but not for the
Coast and Geodetic Survey.
That department of the government has
had a man in the Pacific Coast region for
several months conducting experiments
to determine exactly the variations that
actually exist in the weight of a body at
different altitudes. In computing these
differences, the Survey man measures
distances in terms of one ten-millionth of
an inch, and time in one hundred-thousandths
of a second.
Speaking in broad terms, a mass that
weighs 400 pounds at sea level will weigh,
by spring balances, 399 pounds at an
elevation of five miles. A mass weighing
400 pounds at the equator will weigh 402
pounds at the poles, since the poles are
nearer the center of the earth than are
points on the equator, and the poles are
less affected by centrifugal force than
are points on the equator. Besides these
two large factors which affect the intensity
of gravity, there are local causes,
such as the presence of mountains, or of
materials in the earth's crust of more or
less the average density.
Scientists have discovered that the
greater the pull of gravity the slower a
pendulum of a given size will swung,
hence the relative intensity of gravity
can be determined by comparing the rate
of oscillation of a pendulum at different
localities.
The apparatus with which the experiment
is conducted consists of the pendulum
within its case, three chronometers,
a small box containing an electric light
with a shutter that can be made to
flash a light with each second-beat of a
chronometer, and a telescope for observ­
403
ing light flashes as they are returned
from the swinging pendulum. On the
top of the pendulum is mounted a small
mirror, and on the support of the pendulum
is another similar mirror. These
catch the flash of light from the lamp and
reflect it back to the telescope. When the
pendulum is exactly perpendicular, the
reflected light from its mirror exactly
coincides with the reflected light from
the stationary mirror. The pendulum is
known to have a period of slightly less
than one second. The problem is to
ascertain exactly the period of oscillation
by observing the time which elapses between
the moments at which the two reflected
lights coincide exactly. When
this time is determined, the time of a
single oscillation can be computed
readily.
This simple computation, however, is
far from being" the perfected work of the
observer. Even though he has extended
his observations over a period of half an
hour, and has observed the coincidence of
the lights four or five times, and has
taken the average of the readings, he is
far from that degree of accuracy for
which he strives. In the first place, his
$500 chronometer may not have been absolutely
correct. If it were losing only
four seconds a day, of course there would
be a fraction of a second of loss in the six
or seven minutes between the co-incidences
of the lights that marked the location
of the pendulum in a vertical position
at a second-interval. To correct the time
of the chronometer it is connected with the
telegraph instruments as they are sending
the "time" at noon, and the comparison
of indentures made in a line on a revolving
drum shows the variations from
the true time as kept for the Pacific
Coast by the master clock at Mare Island.