5°6 
The Centre of Gravity of the Earth , [September, 
Pole of the Heavens on January ist, b its position on 
February ist, c on March ist, and so on. With a radius of 
2o"*7, a circle described round the movable points A, b, c, 
&c. will give the Nautilus curve, allowing for the arc 
Diagram 3. 
for each month. Thus an arc of 30°, viz. NAM, will give 
m n, the course of the zenith at the North Pole during 
January. 
