Page:Popular Science Monthly Volume 1.djvu/31

Rh large parts of its surface without spills, or that the spills over many such parts must be very short. "When this consideration is attended to, the spillikin corona will be found to have a very complex and remarkable figure.

It is not to be wondered at that, so soon as the corona began to be thought about at all, astronomers were led to believe that it is not of the nature of a solar appendage, but either a sort of halo in our own atmosphere, or else an appendage belonging in some way to the moon. Kepler and Halley and Newton, to say nothing of a host of other astronomers who considered the question during the infancy of modern astronomy, were led to different conclusions, by the comparatively imperfect evidence available in their day. We may pass over the arguments adduced in favor of the three several theories which were in question. Suffice it that, gradually, it was admitted more and more generally that the corona must be some appendage surrounding the sun; and, in comparatively recent times—a quarter of a century ago, or thereabouts—the opinion began to prevail that the corona is in fact the sun's atmosphere.

But quite recently discoveries were made which seemed to throw great doubt upon this opinion. By means of the instrument called the spectroscope, astronomers have learned not only how to study the sun's colored prominences when the sun is shining in full splendor, but also to determine to some extent the condition of the glowing gas of which those prominences are formed. When this was done, it did not appear that the density of the glowing gas—even close by the sun's body was so great—as might be expected if the corona were an atmosphere properly so called. Some prominences are shown in the figure; and if we consider the pressure to which objects so placed must be subjected, supposing them to lie at the bottom of an atmosphere more than a million miles in height, we shall at once see that the pressure of our own air at the sea-level would be a mere nothing by comparison. It is supposed that our air may be two or three hundred miles in depth, but, even if we suppose it to be ten times as deep as this, the depth of the imagined solar atmosphere would be many times greater. And then the pressure of our air is caused by the earth's attraction, and would be greater if the earth exerted a greater attraction. But the attractive energy of the sun (at his surface) exceeds the force of the earth's gravity about twenty-seven times. "We may safely infer, then, that an atmosphere such as the corona was supposed to be, would cause a pressure exceeding the atmospheric pressure we experience some thousands of times. The gas forming the prominences would be correspondingly compressed under these circumstances. But as a matter of fact the pressure at the very base of the colored prominences appears to be a mere fraction of that which our own air exerts at the sea-level.

Accordingly, Mr. Lockyer, who had taken a prominent part in