Page:Popular Science Monthly Volume 5.djvu/559

Rh the most intricate and capricious patterns the frost ever traces on our window-panes in winter; but, together with this, there was something flame-like in the graceful terminal curves, and something strangely suggestive of fern-like vegetation about the whole. This double and apparently incompatible impression of something at once crystalline and plant-like is strikingly conveyed by many of the penumbral forms, and yet the description will doubtless appear incongruous to any but the few who have seen for themselves. The comparison of the frost-figures is the least inapt, perhaps, to be found, but it is really impossible to obtain an accurate one, when we have no single thing on earth which we can exactly liken it to. When we consider that this extraordinary shape occupied a greater area than the North and South American Continents united while that, over the whole, obtained a temperature far above that of the white flame which plays about the mouth of a furnace, and that its parts turned, as the observer looked, from one evanescent beautiful form to another, with a rapidity of change which indicated the existence of inconceivable force, we need feel less surprise that any metaphor, necessarily drawn from our limited terrestrial analogies, should so fail to convey an adequate idea of what the writer is certain he has seen, but confesses he cannot properly describe.

The umbra or dark inner shade, commences as abruptly as the penumbra, but the contrast between it and the penumbral edge is far greater than between that and the photosphere. We possess no very accurate photometric determinations of the relative light of these portions of the spot, and nothing seems practicable beyond a rough averaging where the umbræ are themselves of such various tints. If we adopt the somewhat crude determinations of the elder Herschel, we may assume that the penumbra is, as a whole, rather less than half as bright as the photosphere, and the umbra about one-seventieth of the brightness of the penumbra. More accurately, if we represent the average brightness of the photosphere by 1,000, that of the penumbra will be denoted by 469, and that of the umbra by only 7. The umbra appears, at first sight, to be black, but this is only from contrast with the superior brightness around it. It is certain, for instance, that sunlight is at least 200,000 times brighter than moonlight (probably more). The umbra, then, if it be but seven thousandths of the brightness of the surface, is still 1,400 times (at least) as bright as the moon, or far brighter than the calcium-light. The absolute depth of the inner edge of the penumbra, below the surface, is not very great, according to M. Faye, and probably not over from 2,000 to 4,000 miles. (Every thing is relative; and, on the sun, 2,000 miles is little for the depth of a cavity which may be from ten to twenty times this width.)

Somewhere about this lower level commences the umbra, which has been already compared to the spout of the funnel, of which the penumbra formed the upper shallow cone; and, through these umbral shades, the eye looks down to quite unknown depths.