314 SCOTT’S LAST EXPEDITION 
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 differ- 
ence may be considered 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 measurements are 
of the utmost refinement and delicacy, comparative measurements 
are nearly always undertaken, and referred to similar observa- 
tions at the standard station in Potsdam. 
The pendulums used were of Col. von Sterneck’s pattern and 
are gilt, three in number, swinging from agate planes in two 
directions at right angles to one another. The pendulums are 
swung at atmospheric temperature and pressure, and corrections 
are applied to reduce to normal. 
The time of swing is measured by the method of coincidences, 
with reference to a special clock making an electric contact each 
second. ‘This clock is used as the standard and rated (if possi- 
ble) to %oth second in the day, by observation of stars at 
meridian transit. 
With care the value of ‘ g’ should be accurate, after all cor- 
rections are applied, to one part in a million, but under the par- 
ticularly unfavourable conditions in the Antarctic it is doubtful 
if a much higher accuracy than 1 in 250,000 is obtainable. 
The difficulties experienced with the instrument in the Ant- 
arctic, though apparently trifling at this distance, were very real 
at the time. For instance, observation on stars for clock rate 
was usually complicated by a temperature of — 40° with a slight 
wind, quite sufficient to keep one nursing his nose, and to be very 
careful not to put one’s eye to the telescope lest it freeze and 
remain there. Other little troubles, such as the stopping of the 
clock by a bodily shift of the wall of the hut upon which it was 
hung, also tended to reduce the accuracy of the observations. 
