450 ASTROPHYSICS 



a great sea of uncondensed vapors, very much as do our terrestrial 

 clouds; but it seems probable that the process of formation is con- 

 tinuous and rapid; and that they are added to from above, or from 

 the interstices, and melt away from below. 



The sun-spots are the most extensively studied and the least under- 

 stood of all solar phenomena. That they are large-scale interruptions 

 in the photosphere, and at the same time the most striking evidence 

 of atmospheric circulation, there can be no doubt. Observations 

 made near the sun's limb, to determine whether the spots are eleva- 

 tions or depressions with reference to the photosphere, seem not to be 

 reliable, perhaps because of abnormal refractions in the strata over- 

 lying and surrounding the spots. In the earth's atmosphere, a high 

 barometer is the indication of descending currents, which generate 

 heat by compression and prevent cloud formation. Is not the umbra 

 of a spot an area of high pressure, which forces the solar atmosphere 

 slowly downward, preventing cloud formation in that area, but 

 favoring the growth of brilliant faculse and flocculi in the regions of 

 uprush surrounding the spot, a theory first suggested by Secchi? 



The visible spots are not the sole evidences of circulation. The 

 surface is covered with a network of interstices, or vents between 

 clouds, which probably exercise all the functions of the visible spots, 

 but on a smaller scale. 



There is no reason to question the truth of Young's discovery that 

 the Fraunhofer lines originate in the absorption of a reversing layer l 

 a thin stratum of uncondensed vapors lying immediately over 

 and between the photospheric clouds. 



The chromospheric stratum, several thousand miles in thickness, 

 includes and extends far above the reversing layers, and contains the 

 lighter gases, such as hydrogen and helium, and the vapors of calcium, 

 sodium, magnesium, and other elements which do not condense under 

 existing temperatures. 



The prominences have in general the same composition as the 

 chromosphere. In some the lighter gases, and in others the heavier 

 metallic vapors, predominate. They are portions of the chromosphere 

 projected beyond its usual level by the more violent ascending cur- 

 rents, or perhaps by eruptions of a volcanic character; and these 

 forces are almost certainly augmented by the pressure of the sun's 

 radiation. It is difficult to account for the quiescent, cloud-like 

 prominences in regions far above the chromosphere on any supposition 

 other than that they are in equilibrium under the opposing influences 

 of gravity and radiation pressure. 



The nature of the forces which control the general and detailed 

 coronal forms is but little understood. Motion within the corona has 

 never been directly observed. Yet we cannot question that the com- 

 1 U. S. Coast Survey Report, 1870, pp. 141-156. 



