4S6 
SCOTT'S LAST EXPEDITION 
(carried on in an ice cave at constant temperature) gave 
a value very slightly lower than that found over the 
sea, and showed, with a self-recording instrument, no 
variations greater than the probable errors of observation. 
For further details of the above, and in respect to 
measurements of the ionisation of the air, the reader is 
referred to the paper before mentioned. 
Samples of sea water were also collected from various 
depths for radium analysis, and will be worked out by 
Professor Joly, but the results are not yet available, 
(c) Pendulum observations. 
It will be known to the general reader that the weight 
of any substance as measured by the pull of the earth 
upon it is not an invariable constant. Thus a piece of 
lead or other substance weighed at sea level on a spring 
balance would be heavier at the Pole than at the Equator 
by about 5 parts in 1000. This difference may be con- 
sidered as partly due to the weakening, as one goes towards 
the Pole, of the centrifugal force of the earth's rotation, 
and partly due to the increased force of attraction by 
virtue of the flattening of the earth at the poles, and 
consequent shorter distance from the centre of attraction. 
Dealing with such small differences in value of the 
gravity constant ' g,' it becomes essential, if any theories 
are to be tested, that observations should be carried out 
with the most extreme accuracy. 
The universal method of measuring ' g ' is by noting 
the time of swing of a pendulum, and as absolute measure- 
ments are of the utmost refinement and delicacy, com- 
parative measurements are nearly always undertaken, and 
