The earliest accurate observers of total eclipses with the telescope noticed that dur ing the total phase red cloud-like masses were seen here and there projecting beyond the limb of the dark moon. Moreover, at the beginning or the end of the eclipse, it is found that these projections are connected with a red border ex tending round the sun. There is, therefore, an envelope which radiates red light and surrounds the sun, and which is invisible except during eclipses. Quite independent of this envelope is a bright effulgence which is seen during a total eclipse. These phenomena are fully described in the article ECLIPSE. What we have now to do is to set forth what they indicate.
The red envelope which rests immediately on the photosphere is called *chromosphere.* It is comparatively thin—so thin as to be almost im mediately covered when the sun is totally eclipsed. Its nature was first made known by the spectroscope, which showed it to be com posed mostly of hydrogen, helium, and calcium vapor. Its principal and lower parts differ in constitution. At the photosphere it comprises nearly all the substances which exist in the latter. This was shown in a very beautiful way by observations of the reversing layer, first made by Young at the total eclipse of 1870. The ex planation of the phenomena there described is that the photosphere is hot enough to shine by its own light, and, being a gas, to give bright spectral lines. But the photosphere is so much hotter than the chromosphere that the latter is, in comparison, a cool gas which absorbs the spectral lines from the light radiated by the photosphere. The question of the density of the chromosphere and reversing layer, as its base is called, has given rise to very varied esti mates.
The fact that the spectroscope shows bright lines as the last ray of true sunlight disappears at the beginning of a total eclipse shows that the gas from which these lines emanate must be so rare as to be transparent through a dis tance of thousands of miles. We are, there fore, justified in concluding that the gases of the chromosphere are extremely rare, and the same is probably true of the principal regions of the photosphere.
Among the most extraordinary phenomena exhibited by the sun are the mountainous eleva tions of the chromosphere, which we see as the red protuberances already described. These are of two kinds, the eruptive and the cloud-like.
The latter present to us the appearance of vast clouds floating in an atmosphere of the sun. It seems certain, however, that they cannot be what they seem, because there can be no atmosphere there to support them. They are probably held up by an impulsion of the solar rays, which will be described presently. The eruptive promi nences seem to be due to outbursts of intensely hot gases, mostly hydrogen, from the sun. These are thrown up with a velocity of several hundred miles per second, like immense moun tains of fire. They sometimes rise to a height of many thousand miles, their ascent being doubtless aided by the impulsion of the solar rays; then they fall back again to the sun. The chromosphere and prominences can now be pho tographed in projection against the sun's disc with the spectroheliograph. When such photo graphs are made with the light of the red hy drogen line, they show great vortex phenomena, centering in sun spots and closely related to the vortices in the photosphere which constitute the spots themselves. See Plate III.
The violent forces seen in action in the chromosphere are in singular contrast to the soft white light of the corona. Much mystery still surrounds the constitution of the latter. It was supposed to be an atmosphere of the sun; but this view is rendered untenable by the fact that an atmosphere supported by its own weight would more than double in density for every mile that it was nearer its base. It probably consists of exceedingly minute molecules of gaseous matter, similar to those which malce up the tail of a comet, and possibly having some resemblance to the latter. The newly-discov an electrical thermometer. The unit usually employed is the calory, which is the amount of heat reqttired to raise one gram of water 1° C. The solar constant is then the number of calories which would be received on each square centimeter of the earth's surface, ex posed perpendicularly to the solar rays, if there were no loss in transmission through the atmosphere.
The following small table gives the mean values of the solar constant obtained from ob servations made at Washington, Mount Whit ney and Mount Wilson. Observations were made at the second station, whose deviation is nearly three miles, in order to test the accu racy of the laws assumed for the absorption of solar rays by the atmosphere.