3. A heavy, seconds pendulum from the Imperial and Royal
Military-Geographical Institute, Vienna.
4. A light, seconds pendulum from the Imperial and Royal
Military-Geographical Institute.
5. A 1/2-second, reversible pendulum of the Geodetic Institute
procured in 1892.
Work was begun in 1898, and in 1906 Kuehnen and Furtwaengler published
their monumental memoir, "Bestimmung der Absoluten Groesze der
Schwerkraft zu Potsdam mit Reversionspendeln."
The acceleration of gravity in the pendulum room of the Geodetic
Institute was determined to be 981.274 +- 0.003 cm/sec^{2}. In view of the
exceptionally careful and thorough determination at the Institute,
Potsdam was accepted as the world base for the absolute value of the
intensity of gravity. The absolute value of gravity at some other
station on the Potsdam system was determined from the times of swing of
an invariable pendulum at the station and at Potsdam by the relation
(T_{1})^{2}/(T_{2})^{2} = g_{2}/g_{1}. Thus, in 1900, Assistant G. R.
Putnam of the Coast and Geodetic Survey swung Mendenhall pendulums at
the Washington base and at Potsdam, and by transfer from Potsdam
determined the intensity of gravity at the Washington base to be 980.112
cm/sec^{2}.[101] In 1933, Lt. E. J. Brown made comparative measurements
with improved apparatus and raised the value at the Washington base to
980.118 cm/sec^{2}.[102]
In view of discrepancies between the results of various relative
determinations, the Coast and Geodetic Survey in 1928 requested the
National Bureau of Standards to make an absolute determination for
Washington. Heyl and Cook used reversible pendulums made of fused silica
having a period of approximately 1 second. Their result, published in
1936, was interpreted to indicate that the value at Potsdam was too high
by 20 parts in 1 million.[103] This estimate was lowered slightly by Sir
Harold Jeffreys of Cambridge, England, who recomputed the results of
Heyl and Cook by different methods.[104]
[Illustration: Figure 31.--MAP SHOWING THE DISTRIBUTION of gravity
stations throughout the United States as of December 1908.]
[Illustration: Figure 32.--MAP SHOWING THE DISTRIBUTION of gravity
stations throughout the United States in 1923.]
In 1939, J. S. Clark published the results of a determination of gravity
with pendulums of a non-ferrous Y-alloy[105] at the National Physical
Laboratory at Teddington, England, and, after recomputation of results
by Jeffreys, the value was found to be 12.8 parts in 1 million less than
the value obtained by transfer from Potsdam. Dr. Hugh L. Dryden of the
National Bureau of Standards, and Dr. A. Berroth of the Geodetic
Institute at Potsdam, have recomputed the Potsdam data by different
methods of adjustment and concluded that the Potsdam value was too high
by about 12 parts in a million.[106] Determination of gravity at
Leningrad by Russian scientists likewise has indicated that the 1906
Potsdam value is too high. In the light of present information, it
appears justifiable to reduce the Potsdam value of 981.274 by .013
cm/sec^{2} for purposes of comparison. If the Brown transfer from
Potsdam in 1933 was taken as accurate, the value for the Washington base
would be 980.105 cm/sec^{2}. In this connection, it is of interest to
note that the value given by Charles S. Peirce for the comparable
Smithsonian base in Washington, as determined by him from comparative
methods in the 1880's and reported in the _Annual Report of the
Superintendent of the Coast and Geodetic Survey for the year 1890-1891_,
was 980.1017 cm/sec^{2}.[107] This value would appear to indicate that
Peirce's pendulums, observations, and methods of reduction of data were
not inferior to those of the scientists of the Royal Prussian Geodetic
Institute at Potsdam.
Doubts concerning the accuracy of the Potsdam value of gravity have
stimulated many new determinations of the intensity of gravity since the
end of World War II. In a paper published in June 1957, A. H. Cook,
Metrology Division, National Physical Laboratory, Teddington, England,
stated:
At present about a dozen new absolute determinations are in
progress or are being planned. Heyl and Cook's reversible
pendulum apparatus is in use in Buenos Aires and further
reversible pendulum experiments have been made in the All Union
Scientific Research Institute of Metrology, Leningrad (V N I I M)
and are planned at Potsdam. A method using a very long pendulum
was tried out in Russia about 1910 and again more recently and
there are plans for similar work in Finland. The first
experiment with a freely falling body was that carried out by
Volet who photographed a graduated scale falling in an enclosure
at low air pressure. Similar experiments have been completed in
Leningrad and are in progress at the Physikalisch-Technische
Bundesanstalt (Brunswick) and at the National Research Council
(Ottawa), and analogous experiments are being prepared at the
National Physical Laboratory and at the National Bureau of
Standards. Finally, Professor Medi, Director of the Istituto
Nazionale di Geofisica (Rome), is attempting to measure the
focal length of the paraboloidal surface of a liquid in a
rotating dish.[108]