1868.] 



Constitution of the Sim and Stars. 



45 



Bat this temporary effect of its vertical motion will be lost, and it will 

 again, when it ceases to ascend and advances only horizontally, direct its 

 course to the north-west, until, in higher latitudes, the swifter rotation 

 which its westward direction imparts to it no longer offers any sen- 

 sible impediment to its sinking in the atmosphere *. And here it will 

 be induced to do so by meeting in greater concentration the upper polar cur- 

 rent, coming too from regions of slower rotatory motion, and therefore with 

 a tendency to descend. Here, then, will end the space throughout which 

 it has split the polar current into two sheets. It descends to the surface 

 of the photosphere producing a zone of variable winds — in this case, how- 

 ever, caused by polar and equatorial currents which are both descending, 

 and so unable to give rise to cyclones. Between this and the pole the 

 equatorial current seems to be next the photosphere, and blows somewhat 

 towards the pole, but chiefly from the east. There will be ascending cur- 

 rents about the poles ; but they will breath upwards so gently, and over so 

 great a space, that they are probably unequal to the task of heaving up 

 the upper stratum of clouds in the vigorous way that leads to cyclones. 

 Our conclusions may now be collected. The annexed diagram shows in 

 one view the directions of the various trade-winds that seem to blow over 

 the surface of the photosphere, and the prevailing character of the zones 

 that separate them. The diagram is in the position in which the sun's 

 disk is usually seen in a telescope. 



76. We found that there is in the inner atmosphere a slight tendency to 

 produce surface-currents from the equator towards the poles, owing to the 

 greater escape of heat at the poles. But the influence of the winds in de- 

 termining currents in the photosphere over which they sweep is probably 

 so predominant, that both between and beyond the belts of spots they 

 are able to determine the currents in the photosphere, those of middle 

 latitudes being accordingly currents towards the east, whilst the equatorial 

 and polar currents set in the opposite direction. Such currents would 

 evidently conspire with the winds that blow over them to produce agitations 

 in the photosphere. They would also contribute to that proper motion 

 of the spots in longitude which has been observed. 



77. "We appear to be compelled to resort to some external cause to account 

 for the periodicity of the spots. Among causes known to exist, that which 

 seems to offer itself with most plausibility for our acceptance is a swarm 

 of meteorites like those which visit us in November three times in a cen- 

 tury, and those which visit us in other months every yeart. To account 

 for the periodicity of the spots, we must suppose the meteorites to describe 

 their orbit in 11*11 years, the period of mutations of the spots. Hence 



* Near the equator a swifter rotation tends to make a body fly outwards through the 

 atmosphere ; near the pole it tends to made the body retreat from the pole without much 

 changing its level. 



f [I find that Sir John Herschel in 1864 suggested such a swarm of meteorites ope- 

 rating in a different way, as a possible cause of this phenomenon. (See Quarterly 

 Journal of Science, 1864, vol. i. p. 233.)— July 1868.] 



