SPOTS ON IT. 



solar orb than the mere fact that the disc of the sun is always 

 circular. It is barely possible, however improbable, that a flat 

 circular disc of matter, the face of which should always be pre- 

 sented to the earth, might be the form of the sun ; and indeed 

 there are a great variety of other forms which, by a particular 

 arrangement of their motions, might present to the eye a circular 

 appearance as well as a globe or sphere. To prove, then, that a 

 body is globular, something more is necessary than the mere fact 

 that it always appears circular. 



1 1 . "When a telescope is directed to the sun, we discover upon it 

 certain marks or spots, of which we shall speak more fully pre- 

 sently. "We observe that these marks, while they preserve the 

 same relative position with respect to each other, move regularly 

 from one side of the sun to the other. They disappear, and 

 continue to be invisible for a certain time, come into view again 

 on the other side, and so once more pass over the sun's disc. 

 This is an effect which would evidently be produced by marks 

 on the surface of a globe, the globe itself revolving on an axis, 

 and carrying these marks upon it. That this is the case, is 

 abundantly proved by the fact that the periods of rotation for all 

 these marks are found to be exactly the same, viz., about twenty- 

 five and a half days. Such is, then, the time of rotation of the 

 sun upon its axis, and that it is a globe remains no longer doubt- 

 ful, since the globe is the only body which, while it revolves with 

 a motion of rotation, could always present the circular appearance 

 to the eye. The axis on which the sun revolves is very nearly 

 perpendicular to the plane of the earth's orbit, and the motion of 

 rotation of the sun upon the axis is in the game direction as the 

 motion of the planets round the sun, that is to say, from west 

 to east. 



12. One of the earliest fruits of the invention of the telescope was 

 the discovery of the spots upon the sun, and the examination of 

 these has gradually led to a knowledge of the physical constitution 

 of the centre of our system. 



When we submit a solar spot to telescopical examination, we 

 discover its appearance to be that of an intensely black irregularly- 

 shaped patch, edged with a penumbral fringe, the brightness of 

 the general surface of the sun gradually fading away into the 

 blackness of the spot. When watched for a considerable time, it is 

 found to undergo a gradual change in its form and magnitude ; at 

 first increasing in size, until it attains some definite limit of mag- 

 nitude, when it ceases to increase, and soon begins, on the 

 contrary, to diminish ; and its diminution goes on gradually, 

 until at length the bright edges closing in upon the dark patch, it 

 dwindles first to a mere point, and finally disappears altogether. 



105 



