12 ROUGH WAYS MADE SMOOTH. 



the margin of the moon's disc it appeared of a reddish hue ; 

 then it changed to a pale yellow, and from the middle to the 

 outer border the yellow gradually became fainter, until at 

 length it seemed almost quite white/ Setting aside the rays 

 and their rotation, d'Ulloa's account of the inner corona 

 may be accepted as satisfactory. The height of this ring 

 was, it seems, about 140,000 miles, or twice that of the ring 

 seen in July 1878. As the year 1779 was one of maximum 

 solar disturbance, there were doubtless many spots in 1778; 

 and the aspect of the corona accorded well with the theory 

 that the corona expands as the number of sun-spots 

 increases. 



We come now to three eclipses which are especially 

 interesting as having been all carefully observed, some 

 observers having seen all three, the eclipses, namely, of 

 1842, 1851, and 1860. Unfortunately the eclipses of 1842 

 and 1851 occurred when the sun-spots were neither at their 

 greatest nor at their least degree of frequency. For a 

 maximum of sun-spots occurred in 1837, and a minimum in 

 1844, so that 1842 was on what may be called the descend- 

 ing slope of a sun-spot wave, nearer the hollow than the 

 crest, but not very near either : again, a maximum occurred 

 in 1848, and a minimum in 1856, so that 1851 was also on 

 the descending slope of a sun-spot wave, rather nearer the 

 crest than the hollow, but one may fairly say about midway 

 between them. Still it is essential in an inquiry of this sort 

 to consider intermediate cases. We must not only apply the 

 cowparentia ad intellectum instantiarum convenient ium, but 

 also the comparentia instantiarum secundum magis ac minus. 

 If the existence of great solar disturbances causes the 

 corona to be greatly enlarged, as compared with the corona 

 seen when the sun shows no spots, we should expect to 

 find the corona moderately enlarged only when the sun 

 shows a considerable but not the maximum number of 

 spots. And again, it is conceivable that we may find some 

 noteworthy difference between the aspect of the corona 

 when sun-spots are diminishing in number, and its aspect 



