42 



Mr. G. J. Stoney on the Physical [Recess, 



out spectral rays of the second order, and no sooner changes its condition 

 than it absorbs through its new spectral line3 heat to such an extent that 

 it must fall back again. Such struggles may be the prolific source of storms 

 when they are local, and perhaps of an appreciable variation in the bright- 

 ness of stars when they are on a great scale. 



Section VI. — Of the Distribution and Periodicity of the Spots. 



72. We may catch a glimpse from the foregoing investigations of what 

 appears at least a possible explanation of several phenomena of the solar 

 spots, which we do not seem yet in a position to refer to their causes with 

 confidence, such phenomena as the local distribution of spots and their 

 periodicity. If from any cause a portion of the lower strata of the outer 

 atmosphere is thrown upwards, it will carry a part of the second stratum 

 of clouds above its natural level. The intermingled air will dilate and tend 

 to cool down as it ascends ; but its temperature will be restored by the heat 

 absorbed and communicated to it by the cloud carried with it. Its thus 

 remaining hot will convert what was perhaps at first only a gentle upheaval 

 into a violent upward current, which will, from the operation of causes fa- 

 miliar upon the earth's surface, occasion a cyclone in the lower strata of 

 the outer atmosphere. The inner atmosphere (that is, the dense atmo- 

 sphere from the surface of the photosphere downwards) cannot be readily 

 drawn into the vortex, by reason of its great specific gravity ; but it will be 

 swept round and round by the violence of the hurricane above, and a kind 

 of whirlpool will result which will depress the central parts into the pen- 

 umbra and umbra of a spot and lift its borders into faculae. The forma- 

 tion of this whirlpool will be greatly assisted if, as we shall presently see 

 we have reason to suspect, there are preexisting currents in the inner atmo- 

 sphere setting in opposite directions along the zones of spots. 



73. If, then, we are right in attributing a large proportion of the spots 

 to ascending currents in the outer atmosphere, we must next seek some 

 cause which can determine the existence of such upward currents in two 

 bands parallel to the equator. It is natural to look for this in some phe- 

 nomenon analogous to our trade- winds ; and, as Sir John Herschel has 

 observed, such a phenomenon may arise if the ellipticity of the sun bring 

 about an unequal escape of heat from his poles and from his equator. The 

 elliptic strata of the atmosphere could be in equilibrio only on the suppo- 

 sition that they are of precisely the same density throughout : but this 

 they cannot be; for as the outer atmosphere is an imperfectly conducting 

 plate, heated on the one side by the photosphere, cooled on the other by 

 radiation towards the sky, at the poles, where the plate is thinnest, its 

 outer strata will be sensibly hotter than their average temperature over the 

 whole sun, and their inner strata very slightly cooler ; and at the equator,* 

 where the plate is thickest, its inner strata will be hotter than the average, 

 and its outer strata cooler. Hence at the poles, where the temperature of 

 the outer parts of the atmosphere is higher than the average, t hey will diffuse 



