172 PROCEEDINGS OF SECTION A. 
and that these and other uncertainties can be partly traced to 
local variations of the force of gravity ; consequently the study of 
these variations may not only assist in improving geodetic results, 
but may also throw light on the geological conditions of the earth’s 
crust to which the variations of gravity are no doubt related. 
The figure and size of the earth are by no means settled 
questions. We may know the length of its major and minor axes 
within a few hundred feet ; but if we consider that upon these 
data depends the accuracy of our astronomical constants and 
magnitudes of the solar system, it seems very desirable to exhaust 
all possible means of research to increase this accuracy. 
More extended gravity observations, it is admitted, would be 
of great service in further testing and probably improving the 
value of the ellipticity or the flattening of the earth at the Poles, 
and the method is more promising than others dependent on 
geodetic ares, on precession and nutation, and on perturbations of 
the moon. We have in the colonies splendid facilities to carry on 
gravity operations along a meridian of some 30° in length, the 
results of which would form a magnificent Australian contribu- 
tion towards the determination of the figure of the earth. 
Since the days of Bouguer Borda and other celebrated experi- 
menters and inv estigators, accurate gravity determinations have 
been made, hitherto generally by pendulum observations, under 
two distinct methods, namely, the absolute method, which gives 
the absolute value of the force of gravity at any given place, and 
the differential method, which gives only its variation from place 
to place. 
The instruments used have varied from the nearest approach 
to the ideal pendulum, as a heavy symmetrical mass attached to 
a thin thread, to the convertible, reversible, and invariable 
pendulums of Bessel and Kater. 
It would be hardly appropriate at this late stage to enter fully 
upon the many and ‘extremely interesting questions which arose 
from a discussion of the results obtained in various parts of the 
world, especially England, India, and Russia ; and I shall there- 
fore pass on with all possible directness to the present and future 
aspect of the case. 
During the past few years a well-marked renewal of interest 
in gravity work has taken place in Europe and America, which, 
I consider, is partly due to the introduction of less ponderous 
instruments, and improved methods of observation, which afford 
greater facilities, conveniences, and rapidity in the execution of 
gravity surveys. 
Extensive operations have been carried on especially in Austria, 
America, and France ; pendulums have been swung at the prin- 
cipal sea-ports more usually frequented by warsbips ; and activity 
is gradually extending. 
