508 The Centre of Gravity of the Earthy [September, 
the polar distance during the middle of January ought 
to be slightly less than on the ist and 31st of January. 
Recorded observation gives 1" for this decrease. 
During February the pole moves from n to o, consequently 
obliquely away from the Pole Star. Draw a line from N 
away from the Pole Star, and from o draw a line at right 
angles to the line joining o and the Pole Star, and where 
this line intersects the line from N we obtain a point the 
distance of which from n will give the change in polar 
distance of the Pole Star during February. This measured 
distance is about 4" # 3. Recorded observation gives 4''*3. 
During April the pole moves from p to Q ; that is almost 
direCtly away from the Pole Star. Measure the arc p q, and 
this arc by scale will give slightly more than 10" as the 
amount by which the polar distance of the Pole Star in- 
creases during April. On referring to the “ Nautical Alma- 
nac ” it will be seen that between March 31st and April 30th 
the polar distance of the Pole Star increases 9''*i. 
Following the curve we find that between r and s — that 
is, about the middle of June — the pole is moving at right 
angles to the arc then joining the pole and the Pole Star, at 
which date the pole cannot vary its distance from the Pole 
Star. On examining the “ Nautical Almanac ” (1880) it 
will be seen that on the 18th of June the pole does not vary 
its distance from the Pole Star. We may now test the 
amount of change in polar distance of the Pole Star between 
June 18th and the end of January, when a line from that 
part of the curve indicated by 18th June to the Pole Star 
cuts the arc N m near N. The length of this arc from scale 
gives 31", which is the increase in polar distance of the Pole 
Star between January 29th and June 18th. Observation, as 
recorded in the “ Nautical Almanac,” gives 31'' for this 
change. As the curve traced by the pole moves round to 
s, T, u, &c., the pole decreases its distance from the Pole 
Star, and from v to w during October moves diredtly towards 
the Pole Star. The arc now traced each month is a longer 
arc than is the arc traced during the early months of the 
year. Consequently when we measure the arc v w from 
scale we find it slightly more than 11 by which amount the 
polar distance of the Pole Star must decrease during October. 
The recorded observations in the “ Nautical Almanac ” give 
n"*3 for the decrease during October. 
We now refer to the monthly changes in right ascension 
as regards the Pole Star. When the pole is carried by the 
annual rotation from m to n the same effect on the right 
ascension is produced as though the star moved over a 
