Rotation of the Planet Saturn upon its Axis . 
59 
Determination of the Period. 
I shall now enter upon the method, which has been used to 
determine the rotation of the planet, from these observations. 
Let K, T, Z, P, in the annexed figure; (see Tab. IX.) repre- 
sent the quintuple belt on the southern hemisphere of Saturn ; 
the different parts of which are diversified as expressed by the 
different tints of the belts : those at EAKNIHMLG 
being uniform, while others at T B C D V, and RQ Y F 
have the southmost of the small dark belts very faint, and the 
northmost pretty strongly marked. Let a be the south pole of 
Saturn, and let the circle 90, 180, 270, 360 represent its equa- 
tor, divided into degrees ; so that a . 180, a 210, a 240, a 270, 
&c. are meridians of Saturn, which, as the planet turns upon its 
axis, from west to east, will successively pass over the line 
a $ y, representing that meridian on Saturn which passes 
through the earth. 
Then the eye of the observer being placed in the line o&(3 y, 
at a great distance, and the hemisphere of Saturn, which is 
here projected on the plane of the equator, being in an oblique 
position, will only see the semicircle $ (3 e. But on account of 
the great inclination of the arches f <5) vj s, to the visual ray 
y /3 a, the eye will not perceive minute divisions, or marks, till 
they come within the limits £ vj, and even then will no where 
judge so well of their brightness and figure as when they draw 
near the situation jQ. 
The divisions on the equator, are to serve us to point out 
those places of the quintuple belt, which, by future calcula- 
tions of the motion of this equator, will be shewn to have been 
on the meridian, u @ y, at any given time ; and the numbers 
I 2 
