Physics of the Earth - II The Figure of the Earth Bulletin of the National Research Council (1931) / Chapter Skim
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Chapter X. Determination of
Chapter X. Determination of
Pages 151-166
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From page 151... ...
oil the earth upon a unit mass on its surface varies from place to place. This -['act was accidentally discovered in 1671 by the French astronomer, Jean Richer, who found that his astronomical clocl<, which had been care:t'ully regulated in Paris, lost 2~ minutes a day on the Island of' Cayenne, South America, and required a shortening, of its pendulum o-l' more than 2 millimeters to malice it keep correct time at the latter place.
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From page 152... ...
theoretical simple pendulum having, the since period, end the intensity of' :,ravitv could be computed groin this le~:~,t,l-~ and the observed period of:' the penclulum. Improved types of the reversible pendulum were later developed as, -for example, the Repsolcl pe~dulu~n Lehigh by means oil-' a hollow shell at the opposite end from the bob was made syn~netrical at the two ends so that the air resistance woulcl be the singe wl:~eu the penclulu~n was swells on either knife edge.
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From page 153... ...
in fi-u-~:e-oi'-the-ea.rtl-~ investiga.tio~-~s where the variation of' ¢,r..~vitv ~.i.tl~ hate of latitude is tile ~->rincipal rector to lie considerecl in clerivin~, the shape of' the eartl~. Nearly every country engaged in measuring the intensity of ~t,ravitv has its own base station whose value has been cleterm;~ed with more or less care by the re]
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From page 154... ...
MODERN GRAVITY APPARATUS There are two general types of modern gravity apparatus for use on land. The older of these two types is 1 |
From page 155... ...
A ~ the dummy pendulum gab ~ thermometer ~tt~cbed to as ~em. ~ is one of the principal pendulums us it appears Ruben Ida OD the knife edge.
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From page 156... ...
The most nearly perfect knife edge that can be made will be found when tested to have what has been called the missing triangle, that is, the two planes forming the edge will not quite meet. The base of this missing triangle on the knife edges of the Coast and Geodetic Survey'~,ravity apparatus has been found by optical means to be less than one micron in length.
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From page 157... ...
It is simply a, special form of the Michelson interferometer adapted-to the purpose of measuring, the flexure or horizontal movement of the kni:t'e edge caused by the horizontal component of the stresses induced by the swinging, pendulum. It hats a strong source of light of Knowles wave-len`~th a.ncl two accurately ground plane-glass reflectors, one on the interferometer itself' and the other attached by means of a, light aluminum arm to the l~nife-edge plate in the receiver.
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From page 158... ...
FIC~ 0F r~ ~6 T~ ~S~:ES~:E~ESE:E.E:ESE:ESESES~:E:ESE:ESE:ESESESESE:E.E.ESESE.E:E:ESE:E.ESE.E.E.E:ESE FIG. 2.-[eDdulum receiver bud interferometer, Coast and Geodetic Survey upp~utus.
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From page 159... ...
The gravity pendulum swings in a. partial vacuum, under a pressure of less than a tenth of ad atmosphere, and is supported on a very accuratelv ground knife edge.
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From page 160... ...
COINCIDENCE OBSERVATIONS The gravity pendulum is a free-swinging pendulum. It has no, directly connected mechanism to drive it or count its oscillations, as that would seriously affect its accuracy.
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From page 161... ...
At that time the two images of the illuminated slit in the flash box, one reflected from the stationary mirror in the receiver and the other from the moving mirror on the pendulum, appear in the telescope in the same horizontal line. The pendulum is.
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From page 162... ...
VON- STERNECK PENDULUM APPARATES The VOll Sternecl: apparatus is quite different in many of its details from Blat of' the Coast and Geodetic Survey, although similar in several essential respects, such as in the length of its pendulums. The pendulum support consists of a well-braced frame secured to a heavy circular base plate which rests on foot screws.
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From page 163... ...
on account of its greater simplicity. The von Sterneck apparatus has the distinction of being the first to be used for accurate gravity determinations at sea.
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From page 164... ...
Many attempts were made, by a number of scientific men o-t' different countries using several different kinds of specially designed apparatus to meet this lack but the results were of little geophysical value. IJntil recently it seemed impossible to obtain an accuracy at all comparable with that of land determinations.
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From page 165... ...
Besides the three principal pendulums there are two short, heavily damped pendulums, one of which records the tilt of the apparatus, at right angles to the plane of' oscillation and thus takes account of the endwise acceleration of the ship, and the other serves as err object of reference in recording the a.mplitude of oscillation of the middle one of the principal pendulums. These data are needed in making the computations.
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From page 166... ...
that those interested in making a more detailed study of the different kinds of pendulum apparatus and methods used in determining gravity, consult the following books and articles: Fran~,ois, Ch. Note sur l'appareil gravimetrique de l'Observatoire Royal de Belgique.
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