THE SUN. 



The period which elapses between the formation of the spot, its 

 gradual enlargement, subsequent diminution, and final disap- 

 pearance, is very various. Some spots appear and disappear very 

 rapidly, while others have lasted for weeks and even for months. 



13. The magnitudes of the spots, and the velocities with which the 

 matter composing their edges and fringes moves, as they increase 

 and decrease, are on a scale proportionate to the dimensions of the 

 orb of the sun itself. When it is considered that a space upon the 

 sun's disc, the apparent breadth of which is only a minute, 

 actually measures 27960 miles, and that spots have been frequently 

 observed, the apparent length and breadth of which have exceeded 

 2', the stupendous magnitude of the regions they occupy may be 

 easily conceived. 



14. The velocity with which the luminous matter at the edges of 

 the spots occasionally moves, during the gradual increase or 

 diminution of the spot, has been in some cases found to be enor- 

 mous. A spot, the apparent breadth of which was 90", was 

 observed by Mayer to close in about 40 days. Now, the actual 

 linear dimensions of such a spot must have been 41940 miles, 

 and consequently, the average daily motion of the matter com- 

 posing its edges must have been 1050 miles, a velocity equivalent 

 to forty-four miles an hour. 



15. Two, and only two, suppositions have been proposed to ex- 

 plain the spots. One supposes them to be scoriae, or dark scales of 

 incombustible matter, floating on the general surface of the sun. 

 The other supposes them, to be excavations in the luminous matter 

 which coats the sun, the dark part of the spot being a part of the 

 solid non-luminous nucleus of the sun. In this latter hypothesis 

 it is assumed that the sun is a solid non-luminous globe, covered 

 with a coating of a certain thickness of luminous matter. 



That the spots are excavations, and not mere black patches on 

 the surface, is proved by the following observations : If we select 

 a spot which is at the centre of the sun's disc, having some definite 

 form, such as that of a circle, and watch its changes of appear- 

 ance, when, by the rotation of the sun, it is carried towards the 

 edge, we find, first, that the circle becomes an oval. This, how- 

 ever, is what would be expected, even if the spot were a circular 

 patch, inasmuch as a circle seen obliquely is foreshortened into an 

 oval. But we find that as the spot moves towards the side of the sun's 

 limb, the black patch gradually disappears, the penumbral fringe 

 on the inside of the spot becomes invisible, while the penumbral 

 fringe on the outside of the spot increases in apparent breadth, so that 

 when the spot approaches the edge of the sun, the only part that 

 is visible is the external penumbral fringe. Now, this is exactly 

 what would occur if the spot were an excavation. The penumbral 

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