The pendulum methods of determining A are not comparable in
accuracy with the methods of Cavendish and Poynting. The same
is true of the determination of A from observations on the deflection
of the plumb line or spirit level by a mountain — a method employed
by Bouguer on Chimborazo, in 1740, and Maskelyne on Schehallien,
1774.
d. Study of local variations in g. Value is less than normal in
mountainous regions and greater on islands and borders of conti-
nents. Observations made among the Himalayas by Indian Survey
up to an elevation of 15,408 ft. (Basevi). In Europe by Defforges, in
United States by Dejfforges and Putnam.
OBSERVATIONS OF PUTNAM





Elevation m.
Observed value of g

reduced to

sea level cm.
Computed
value of g cm.
WashinKton


14
1638
4293
2386


980.101.
979.941
979.844
980.369


980.087


Denver


980.156


Pike's Peak


980.083


Yellowstone Can von


980.605







84 NOTES, PROBLEMS AND EXERCISES

e, g is independent of mass and material. (Newton, 1687; Bes-
sel, 1832). Mass and material of bob of pendulimi varied; time of
vibration remains unchanged.
II. Determination of Figure of Earth. (Richer, 1672; Newton,
1687; Clairaut, 1743.)

From the law of variation of g with latitude it is possible to deter-

mine the eUipticity or oblateness of the earth. EUipticity c=.
Clairaut showed (1743) that for a spheroid of equilibrimn of small
oblateness composed of concentric strata each of the same density
throughout, which, presumably, is approximately true of the earth
as a whole, the eUipticity is e=^7— f where y is the ratio at the
equator of the centrifugal f yce to gravity and f the total fractional
increase of gravity from equator to pole. At a much later date
(1849) Stokes showed that for such a spheroid no particular law of
density need be assumed.
