The French astronomers, La Hire and De Lisle, put forward two theories, which are now obviously untenable. According to each theory, the appearance of the corona is caused by an action on the sun's rays, that action taking place at the edge of the moon's disc-the difference between the two theories being that La Hire ascribed the action to the inequalities of the moon's surface and their power of reflecting the solar rays, while De Lisle supposed that the sun's rays were diffracted at the moon's edge. We owe to Baden Powell and Sir David Brewster the disproof of De Lisle's theory, De Lisle himself having disposed of La Hire's.

There remains, then, only one theory to consider, the theory, viz., that the corona is a true solar appendage, and one of the most remarkable features in the universe (for the other, that the corona is simply a terrestrial phenomenon, due to the passage of the sun's rays through our own atmosphere, has never been supported by anyone competent to express an opinion on such matters, and has now disappeared even from the writings of those who advanced it).

The corona cannot be a solar atmosphere. It will be obvious that if the corona were such an atmosphere, it would exert a pressure upon the sun's surface corresponding to that pressure which our own atmosphere exerts upon the surface of the earth. But then the pressure exerted by the coronal atmosphere would be incalculably greater. Our own atmosphere, we have reason to believe, does not extend much more than 100 miles above the sea-level. Now the corona is visible, under favourable circumstances, at a distance from the sun equal to his own diameter-setting aside all considerations of the radial projections. In other words, it certainly does not extend less than 850,000 miles from his surface. Regarded as an atmosphere, therefore, the corona is certainly not less than 8,000 times as deep as our own. On this account alone the pressure it would exert would be enormously greater. For it is to be noted that the pressure

exerted by our air would not be merely doubled were the height of the atmosphere doubled, trebled were that height trebled, and so on, but would increase at a much more rapid rate. If a mine were sunk into the earth in order to measure the increase of atmospheric pressure with depth, instead of a depth of 100 miles being required in order to have a double pressure, only 3 miles would be needed. At the bottom of a mine 7 miles deep the pressure would be four times as great as at the sea-level; 101⁄2 miles deep the pressure would be eight times as great; 14 miles deep the pressure would be sixteen times as great, and so on, like the expense of the miser's grave, 'doubling as we descend' for every 3 miles. It requires no great knowledge of arithmetic to see that the pressure at a depth of 100 miles or so would be millions of times greater than that at the sealevel. It will be seen, therefore, how inconceivably great the pressure exerted by a solar atmosphere some 8,000 times as deep as ours would necessarily be, let the nature of the gases composing it be what it may.

But even this is not all. We have hitherto only compared the height of the supposed solar atmosphere with that of the earth's. We must not forget that the sun's attractive energy so enormously exceeds the earth's that even though his atmosphere were no deeper than ours (and similarly constituted) the pressure exerted on his surface would be enormously increased. If a man could be placed on the solar surface his own weight would crush him as effectually as though while on the earth a weight of a couple of tons were heaped upon him. In precisely the same way the pressure of the solar atmosphere is increased by the enormous force with which the sun drags towards himself every particle composing that atmosphere.

Now it happens that we know quite well that the pressure exerted by the real solar atmosphere, even close by the

1 The actual number representing the proportionate pressure would consist of no less than nine figures, being very nearly two hundred millions.

bright surface which forms the visible globe of the sun, is nothing like so great as it would be if the corona formed part of that atmosphere. The bright lines constituting the spectrum of the coloured prominences would be many times thicker than they are if the pressure were so great; for spectroscopists have found, by means of experiments made in the laboratory, that with increase of pressure the spectral bright lines of a gas increase in thickness.

Here, then, we have the most conclusive proof possible that the corona is not a solar atmosphere.

But, on the other hand, those who argue that the corona is a solar appendage, ask how it happens, if the phenomenon is due to the illumination of our own atmosphere, that the moon looks black in the very heart of this illumination. If our air were illuminated, its light would extend over the moon also-since the moon lies so far beyond its limits ; whereas the moon is as a dark disc on the background of the coronal light. This very word background, obviously applicable to the corona as actually seen, indicates that the source of the coronal light is beyond the moon.

Here, then (to mention no other considerations), we have the most conclusive evidence that the corona is not a phenomenon of our own atmosphere.

But then the corona is clearly somewhere and something. If its light comes from beyond the moon, we need not doubt that it comes from the sun's neighbourhood; and again, if the corona is not a solar atmosphere, we can scarcely doubt that it is a solar appendage. It would seem to follow that the corona is due to bodies of some sort travelling around the sun, and by their motion preserved either from falling towards him (in which case the corona would quickly disappear) or from producing any pressure upon his surface, as an atmosphere would.

Whatever the corona may be, it is clear that regarding it as a solar appendage-a conclusion which seems forced upon us by the evidence-it is presented to us as one of the most striking and imposing of all the phenomena of the solar

system. It is a fitting crown of glory for that orb which sways the planets by its attraction, warms them by its fires, illuminates them by the splendour of its light, and pours forth on all of them the electric and chemic influences which are as necessary as light and heat for the welfare of their inhabitants.

THE SUN'S LONG STREAMERS.

PROFESSOR CLEVELAND Abbe, an American astronomer and meteorologist, who had intended to observe the eclipse of the sun in July 1878 from the summit of Pike's Peak, in Colorado, more than 14,000 feet above the sea-level, fell ill after he had reached that place, and was carried down to the Lake House (elevation 10,000 feet), there to remain while the rest of his party stayed to view the eclipse from the summit. Probably if he had remained with them his observations would have differed in no very marked degree from those which other astronomers made on that occasion. He would have devoted a few seconds, perhaps, to the study of the sun's corona with the naked eye. He would probably have made some telescopic, spectroscopic, or polariscopic observations during the rest of the three minutes during which the total eclipse lasted, and possibly he might have noted some feature rather more effectively and satisfactorily than most of the other observers. But under the actual circumstances he could not hope thus to take his place among the thousands of observers who have noted the phenomena of total solar eclipses. He had no optical or other instrument. Worse than all, he is nearsighted; and though he had a pair of spectacles, it was not quite strong enough to correct his near-sightedness.

Yet Professor Abbe succeeded in making observations far exceeding in interest any which were made by the entire force of eclipse observers in 1874 and 1875, and fairly comparable in this respect with the most remarkable dis