THE 
SCIENCE. 
JOURNAL OF 
SEPTEMBER, 1881. 
I. THE CENTRE OF GRAVITY OF THE EARTH, 
AND ITS 
EFFECT ON ASTRONOMICAL OBSERVATIONS. 
By Col. A. W. Drayson, R.A., F.R.A.S. 
(Concluded from page 448.) 
S HE Nautilus curve traced by each pole can be con- 
structed from the data already known, viz., the North 
Pole changes its direction, due to the change in direc- 
tion of the Earth’s axis, about 2o"*25 annually towards the 
first point of Aries. Whilst the North Pole is tracing this 
line, or arc, the zenith of the pole — found by a plumb-line 
directed to the centre of gravity of the Earth — will trace a 
circle round the movable pole ; consequently, as the zenith 
of an observer at the pole is supposed to be the same point 
in the Heavens as that occupied by the pole, the appearance 
will be as though the pole itself actually traced the Nautilus 
curve, and the effects on stars declinations and right ascen- 
sions will be the same as though the poles traced this curve. 
To construct the curve the following method should be 
adopted : — 
Draw a line (a m, Diagram 3 ) of any convenient length, 
say 2 inches, and construct a scale of seconds of arc, so that 
A m on this scale is 2 o "'25 in length ; the line am represents 
the course of the North Pole towards the first point of Aries 
during any year. Divide the arc am into twelve equal parts, 
to indicate the position of the North Pole on the 1st day of 
each month. Consequently A is the position of the North 
VOL. III. (THIRD SERIES). 2 L 
