116 OUTLINES OF NATURAL FHILOSOPHY. 



#. The secular variation of the equation of time, is 

 different for every different state of that equation : 

 'it is greatest about the time when the sun is in the 

 perigee; and it is then 15".6. 



The Sun's Rotation on his Axis. 



114. The face of the sun, when viewed with 

 a telescope, though of a bright and intense 

 light, far above that of any other object, is of- 

 ten marked with dark Spots, which, when exa- 

 mined from day to day, are found to traverse 

 the whole surface from east to west, in the space 

 nearly of fourteen days. 



a. These spots, though only visible with the telescope, 

 are sometimes so large, as to subtend an angle 

 nearly of one minute. Their number, position, 

 and magnitude, are extremely variable. Each of 

 them is usually surrounded with a Penumbra, be- 

 yond which is a border of light, more brilliant than 

 the rest of the sun's disk. When a spot is first 

 discovered on the eastern limb, it appears like a 

 fine line ; its breadth augments, as it approaches 

 the middle of the disk, from which it diminishes 

 as it goes over to the western limb, where, at last, 

 it entirely disappears. The same spot, after four- 

 teen days, is sometimes discovered again on the 



east 



