158 THE ANTARCTIC MANUAL. 
of saturated steam, or the boiling-point of water, at the sea level, we 
have a method of determining the force of gravity at any place at 
that level. But we must be on our guard. Our knowledge of the 
relation existing between the boiling point of water and the pressure 
on its surface is derived from observations of the barometer. There- 
fore, from the boiling point of water we cannot get absolute data with 
regard to the force of gravity. But it may be that it can afford 
information about the variations of the force of gravity. 
For instance, we observe our standard barometer having the tem- 
perature of melting ice at the sea level, in lat. 20°, 45° and 70°, and 
at each of these places its height is 735°5 mm.; we apply the usual 
correction to reduce this barometric height to the equivalent height 
when the mercury is exposed to the same gravitational force of 
attraction, By convention, the standard force of terrestial attraction 
is taken to be that exerted at the sea level in lat. 45°. The baro- 
metric heights thus adjusted are now 734, 735-5 and 737 mm., and 
these columns of mercury under the influence of standard gravity 
exercise pressures of 1002°04, 1000 and 997°8 grm. per square centi- 
metre. It results from experiments, principally by Regnault, on the 
relation between the boiling point of water and the height of the 
standard barometer, that at these true pressures water boils at 
99°-029, 99°-087 and 99°:142 C. respectively. 
Supposing that we take the hypsometer to these latitudes, and 
that we determine the boiling point of water with every care at each 
latitude at the sea-level when the standard barometer at 0° C. stands 
at 735°5 mm., and we find that the boiling point-is the same, namely 
99°087° C. at all these latitudes, then we conclude that the true 
pressure of the atmosphere at each of the localities is 1000 grm. per 
square centimetre. But we know from the geodetic determination 
of the figure of the earth that the sea-level is nearer the centre in 
lat. 70° than it is in lat. 45°, and nearer in lat. 45° than in lat. 10°. 
If the earth is homogeneous the force of gravity must be greater at 
lat. 70° than at lat. 45°, and at lat. 45° than at lat. 10°. But from 
the identity of the height of the barometer at the three localities we 
know that the mass of the atmospheric column is the same at the 
three places, and from the identity of the boiling point of water we 
know that the weight of the three columns or the atmospheric pres- 
sure is the same in the three places. As the force of gravity depends 
only on the mass of the attracting body and on the distance of its 
centre from the attracted body, and we find that, when this distance 
is varied in a certain measure no alteration is produced in the 
effective force of gravity the conclusion is necessary, that the effect 
