253 
1879.] 
J. T. Walker —On Indian Pendulum Observations. 
considerable disturbance of the sea-level, so that all radii drawn from the 
centre of the earth to the surface of the ocean will be sensibly equal 
assuming the figure to be exactly spherical in order to avoid circumlocu¬ 
tion ; it also follows that deflections of the plumb-line are not likely to be 
very considerable, excepting in the immediate vicinity of mountain masses, 
where the deficiency below cannot neutralise the excess above. If so, then 
distant mountain masses may cease to be regarded as prejudicial to geodetic 
operations, for their influence will be sufficiently counteracted by other 
causes ; the resultant effect at a distance may even be materially less than 
that of local and contiguous irregularities in the configuration of the 
ground, the magnitudes of which may be insignificant as compared with 
those of the mountain and continental masses. In like manner the de¬ 
ficiency of ocean-density need not be regarded as liable to influence distant 
geodetic operations, as it may be expected to be neutralised by an increase 
of density in the crust below the ocean-bed. 
On the other hand, if the hypothesis is not correct, we are driven to 
conclude that the radii of the (spherical) earth are of unequal length, and 
that there must be considerable variations between the apparent level of 
the sea and the normal level which corresponds to the curve of equal radii. 
In this case the actual irregularities of the surface of the earth will be 
much greater than they appear to be, and the greater will be the departure 
of the Actual Figure from a simple geometrical figure, such as the Mean 
Figure—either a spheroid of two axes, or a triaxial ellipsoid—which geode¬ 
sists deduce from their measurements over the earth’s surface. 
This latter view of the subject has found a warm advocate in Germany, 
in the person of Dr. J. Hann, who urges—in a paper published in Vienna, 
in the Mittheilungen der Geographische Gesellschaft, 1875, No. 12—that 
the sea-level is greatly distorted, because of the unequal distribution of 
matter on the earth’s surface ; consequently, that we can no longer hope 
that geodetic measurements, reduced to a sea-level thus distorted, will con¬ 
form to a regular ellipsoid of revolution ; that our knowledge of the true 
form of the earth is deficient; and that it has become desirable to resort to 
pendulum operations, in order to determine the variation of gravity as 
against some normal station—at as many oceanic islands as possible, and 
also at numerous stations on the coasts and in the interior of the great 
continents, in order to discover, by the shortest method, the irregularities 
of the sea-level. • . 
Measurements of the variations of gravity have, unquestionably, one 
great advantage over measurements of meridional and longitudinal arcs, in 
that they can be conducted with far greater rapidity and at a far smaller 
cost. But they are open to a grave objection, in that when reducing them 
