424 THE CHEMISTRY OF THE SUN. [CHAP 



We know indeed that meteorites falling in our own atmosphere 

 explode violently, but how this effect must be enhanced, when 

 the fall of small masses through a low cool atmosphere is 

 exchanged for the falls of hundreds of millions of tons through 

 a high incandescent one, each stratum of which is hotter than 

 the one overlying it, until at length the temperature cannot be 

 denned ! It must not be forgotten that at times the quantity of 

 vapours produced, vapours which are truly incandescent, though 

 they are cooler than the photosphere, is sufficient, whatever be 

 the process of their production, to cover an area of a thousand 

 millions of square miles. 



It is a fact worth pointing out, that if we assume a million tons 

 of iron to be dissociated so as to get the increase of volume 

 indicated in the second table, and to explode in T ^th of a 

 second, the initial velocity of the dissociated particles along 

 the line of least resistance would be something like 200 miles 

 a second. 



8. By the hypothesis, since we know from photographs of the 

 eclipsed sun that the thickness of the corona is greatest near the 

 equator, the nearer a spot is to the equator the more rapid should 

 be its motion across the disc, because the forward (eastward) 

 motion of the 'particles which reach the photosphere is the more 

 rapid before they begin to descend. 



Scheiner who was the first to observe the spots with any 

 very great and continuous care made what appeared to him 

 the paradoxical discovery that the spots which were nearest to 

 the sun's equator appeared to travel at a quicker rate than those 

 which were nearer the sun's poles. This fact, however, was not 

 generally recognised until much later, and the discrepancies 

 between the various periods of solar rotation deduced by different 

 observers, occasioned much perplexity. In reality, they depended 

 simply upon the position on the sun's disc of the particular 

 spot observed for the purpose. The average time in which 



