THE EDGE OF THE SUN — MENZEL 219 



that the "luminous decompositions" responsible for the light "must 

 unavoidably be attended with great agitations, such as with us might 

 even be called hurricanes." Herschel postulated the existence of 

 "empyreal gas," an unstable substance whose decomposition produced 

 the solar radiation. Excess generation of this substance would lead 

 to instabilities of the sun's atmosphere. The gas would burst violently 

 through the intermediate regions, causing openings and sunspots and 

 generally promoting "maintenance of the general luminous 

 phenomena." 



Many years later, John Herschel proposed an alternative theory of 

 sunspots in terms of a general circulation of the solar atmosphere. He 

 knew that terrestrial hurricanes and cylones owe their existence to 

 temperature differences between pole and equator, as I have already 

 noted. He argued that the centrifugal force of rotation would cause 

 the sun to bulge at the equator and thicken the atmosphere. Since 

 Herschel believed that solar radiation originated in the outer layers, 

 his argument indicated an atmosphere hotter at the equator than at 

 the poles. Thus one would expect strong horizontal driving forces, as 

 in the terrestrial atmosphere. The sunspots were to be hurricanes, 

 holes cut by the whirlwind that exposed the cooler layers below. 



C. A. Young, of Princeton, noted that any existing temperature 

 difference should in reality occur in the reverse sense. The poles 

 should be hotter than the equator because they are nearer the center 

 of the sun. He pointed out that the effect must be insensible, how- 

 ever, since there is no detectable difference in brightness between the 

 polar and equatorial limbs of the sun. The spots showed no "system- 

 atic drift north or south as solar trade-winds would necessarily 

 produce." 



Faye and Secchi rejected the horizontal-circulation theory and inde- 

 pendently suggested that spots resulted from gases flowing outward 

 under internal pressure. The proponents of this hypothesis withdrew 

 it when they became convinced that the spectrum of superheated gas 

 should show emission lines rather than the absorption spectrum charac- 

 teristic of spots. The argument is actually fallacious, though it ap- 

 peared to be sound at the time. 



Hale's discovery in 1914 that sunspots possess powerful magnetic 

 fields played an important part in the formulation of our theories. It 

 was immediately apparent that this magnetism must arise from circu- 

 lating electric currents. And, for some obscure reason, scientists con- 

 cluded that these currents could come into being only if the matter 

 itself were circulating. In other words, astronomers revived the 

 concept of sunspots as vortices. Practically every book on the sun 

 written since that time and every textbook on astronomy refers to 

 sunspots as "stormy areas," in every sense analogous to terrestrial 



