1868.] 



Constitution of the Sun and Stars. 



43 



themselves upwards and overflow ; at the equator, where the temperature is 

 less than the average, they will subside and tend to escape laterally at the 

 bottom. Moreover, the lower strata being subjected at the equator to more 

 pressure than the average, by reason of the coolness of the superincumbent 

 strata, and to less pressure at the poles, will also contribute to produce an 

 under-current in the outer atmosphere from the equator towards the poles. 

 Hence if it were not that the rotation of the sun modifies the result, we 

 should have a constant wind blowing steadily from the equator to the 

 poles over the surface of the photosphere, and a counter-current in the 

 upper regions of the atmosphere. 



74. The effect upon the inner atmosphere is directly the reverse. Heat 

 will escape from the photosphere very slightly more freely at the poles, 

 less freely at the equator, than the average. The upper strata of the inner 

 atmosphere will therefore be a little lighter at the equator, and will over- 

 flow towards the poles, tending to produce a feeble surface-current in the 

 photosphere in the same direction as the wind which blows] above it from 

 the equator towards the poles. 



75. But the sun's surface is all the time being carried round by his ro- 

 tation from east to west. This will impart a strong westerly direction to 

 the descending current where it reaches the photosphere at the equator*, 

 and will further render it where it spreads out over the photosphere towards 

 the poles, a south-east wind in the northern hemisphere, and a north-east 

 wind in the southern. Thus these winds blow in such directions as to 

 rotate more rapidly than the general body of the sun, and they therefore 

 seek to raise themseves above the photospheref . At the equator the 



* [This first part of the effect of the sun's rotation was overlooked hy the author 

 when writing this paper. The omission has been supplied in the text above, and the 

 reader is requested to correct the error in the abstract of the memoir (see Proceedings 

 of the Royal Society for June, 1867) where a calm is spoken of as prevailing over the 

 equatorial zone of the photosphere. — July 1868.] 



f The similar centrifugal tendency in the current which overflows from the earth's 

 equator would no doubt keep it throughout its whole course outside the polar current, 

 were it not that being charged with moisture and cooling as it advances, some of its strata 

 soon become so loaded with clouds, that they, and the clouds amongst which they are 

 entangled, come to have between them so much higher a specific gravity that their 

 downward tendency due to this cause overcomes the outward tendency of their superior 

 rotatory motion. In the case of the sun, on the other hand, the clouds and the ro- 

 tatory motion operate at first in the same direction upon the equatorial current, botli 

 tending to raise it ; but after some elevation has been attained, they there, as on the earth, 

 act in opposite directions, the motion of rotation tending to depress the current as soon as 

 it gets above a height which must be moderate, when compared with the vast extent of 

 the sun's atmosphere. 



Furthermore, the earth's polar current coming from regions of slower motion has a 

 tendency to descend, and when it gets down, a tendency to creep along the surface of 

 the ground. Both the currents accordingly seem to contribute to that descent of both 

 which takes place in the earth's temperate zones. But on the sun the ascent of both the 

 currents over the zones of spots seems to be brought about by the tendency of the equa- 

 torial current to rise being more powerful than the tendency of the polar current to cling 

 to the photosphere, 



