RADIATION". 



sun is dispersed through the surrounding space by radiation. If, as 

 may be assumed, the rate at -which this heat is generated be the 

 same on all parts of the sun, and if, moreover, the radiation be 

 equally free and unobstructed from all parts of its surface, it is 

 evident that a uniform temperature must be everywhere 'main- 

 tained. But if, from any local cause, the radiation be more 

 obstructed in some regions than in others, heat mil accumulate in 

 the former, and the local temperature will be more elevated there 

 than where the radiation is more free. 



But the only obstruction to free radiation from the sun must 

 arise from the atmosphere with which to an height so enormous it 

 is surrounded. If, however, this atmosphere have everywhere the 

 same height and the same density, it will present the same obstruc- 

 tion to radiation, and the effective radiation which takes place 

 through it, though more feeble than that which would be produced 

 in its absence, is still uniform. 



But since the sun has a motion of rotation on its axis in 

 25 dt 7 h * 48"", its atmosphere, like that of the earth, must partici- 

 pate in that motion and the effects of centrifugal force upon matter 

 so mobile : the equatorial zone being carried round with a velocity 

 greater than 300 miles per second, while the polar zones are moved 

 at a rate indefinitely slower, all the effects to which the spheroidal 

 form of the earth is due will affect this fluid with an energy pro- 

 portionate to its tenuity and mobility, the consequence of which 

 will be that it will assume the form of an oblate spheroid, whose 

 axis will be that of the sun's rotation. It will flow from the poles to 

 the equator, and its height over the zones contiguous to the equator 

 will be greater than over those contiguous to the poles, in a degree 

 proportionate to the ellipticity of the atmospheric spheroid. 



Xow, if this reasoning be admitted, it will follow that the 

 obstruction to radiation produced by the solar atmosphere is 

 greatest over the equator, and gradually decreases in proceeding 

 towards either pole. The accumulation of heat, and consequent 

 elevation of temperature, is, therefore, greatest at the equator, and 

 gradually decreases towards the poles, exactly as happens on the 

 earth from other and different physical causes. 



25. The effects of this inequality of temperature, combined with 

 the rotation, upon the solar atmosphere, will of course be similar in 

 their general character, and different only in degree from the 

 phenomena produced by the like cause on the earth. Inferior 

 currents will, as upon the earth, prevail towards the equator, and 

 superior counter-currents towards the poles. The spots of 

 the sun would, therefore, be assimilated to those tropical regions 

 of the earth in which, for the moment, hurricanes and tornadoes 

 prevail, the upper stratum which has come from the equator being 



111 



