12 
the bright surface of the sun, and is called the penumbra; a 
darker interior portion called the umbra; and, finally, in the 
umbra itself some still darker parts, to which is attached the 
name of the nucleus. These nuclei were first detected by the 
keen sight of that eminent observer, the late Rev. Mr. Dawes. 
The penumbra is traversed by bright streams of matter, which 
are supposed to indicate the gradual rushing in of the photosphere 
or visible surface of the sun into the spot, ultimately causing its 
absorption, and consequent disappearance. Spots are not found 
all over the sun, but confined to fixed zones extending to about 
80° N. and S. of the Equator. The first deduction from the 
discovery of sun-spots was that of the rotation of the sun upon an 
axis, the mean rotation being accomplished in 25} days, although 
the rate varies somewhat with the latitude. A spot apparently 
advances upon the solar disc from H. to W., and it not unfre- 
quently happens that the same spot, being carried round by the 
sun, will reappear for several successive rotations. 
That most spots are hollows or cavities, and not merely of 
the nature of scorie floating on the surface, is demonstrated by 
the manner in which a spot appears and disappears on the EH. and 
W. limb of the sun respectively. Were the spot a surface 
phenomenon, we should expect to detect first the right or W. 
side of the penumbra, then the umbra, and finally the most 
easterly side of the penumbra. ‘The reverse order would be 
expected at the W. limb, where the spot disappears. The facts 
observed are for the most part in contradiction to any such 
hypothesis. We first see a slight portion, reduced to a mere line 
by perspective, of the EH. edge of the penumbra, then the umbra, 
and finally the W. edge of the penumbra, demonstrating that we 
are looking into a cavity in a spherical surface. Nevertheless, 
there are exceptions to this rule, and the law of foreshortening, 
as stated by Wilson and others, appears to be neither so general 
nor so decided as was by them supposed. 
Among various phenomena connected with sun-spots may 
be mentioned the double penumbra which some spots exhibit, 
the repulsion which others appear to exert on one another, the 
bright ring which is invariably seen between the umbra and 
penumbra of spots, the red veils sometimes detected over them, 
especially when they are large, as also the extremely rapid growth 
with which some spots develope. 
Finally, in this hurried enumeration of the aspects of sun- 
spots, we cannot afford to omit what is perhaps the cardinal 
point in all sun-spot phenomena, connecting them undoubtedly 
with terrestrial magnetism, and probably also with changes of 
weather—namely, the secular variation of the spotted area of the 
sun, the period from maximum to maximum being about eleven 
years. Such a discovery was the reward of the patient toil of 
