thousands of fragments, some of which weighed as much as 17 lbs. This created so much astonishment that the French Government sent M. Biot, a celebrated French chemist, to examine into the matter, and he reported that there could be no doubt that a shower of hot stones had fallen upon the earth.

From this time more interest was taken in meteors and meteoric stones. People had remarked for a long time that shooting-stars were more abundant from the 9th to the 11th of August than at other times, and more lately it was also noticed that a shower of the same kind happens about the 13th of November. Astronomers began, therefore, to think that these meteors must move in regular orbits, crossing the orbit of our earth in certain places, so that we pass through them. There were also reasons for thinking that the November meteors travelled in an enormous ellipse, passing at one end even outside the planet Uranus.

It was not, however, till thirteen years ago that anything was really known. In the year 1862 an Italian astronomer named Schiaparelli made a very remarkable suggestion. He noticed that a comet which was seen in that year crossed the earth's path just at the point where we are always in the middle of the meteor-shower on August 10, and it occurred to him whether it might not be possible that the August meteors were travelling in the same orbit as the comet. His guess turned out perfectly right, and by a calculation which we cannot follow here he proved that the comet and the August meteors travel along precisely the same path in the shape of a long ellipse passing at one end outside the planet Neptune, the most distant of the known planets. This was the first time that the orbit of any set of meteors had been traced out.

The next was that of the November meteors, which was

CH. XXXI.

METEOR-SHOWERS.

2.99

determined by Adams, and also independently by Leverrier. It had been shown by searching out all the past accounts of November showers that in times gone by, the earth passed through these meteors a little earlier in the year than she does now, and this could not be accounted for by any irregularity in the movement of the earth. It looked therefore as if the orbit of the November meteors must be slowly shifting, just as the orbits of the planets do, within certain limits. It was upon this shifting that Adams founded his calculations, and he worked out the meteor path with great accuracy, showing that those astronomers had been right who thought it extended beyond Uranus. This time the problem was solved by pure astronomical reasoning and not by a happy guess. But perhaps the most remarkable part of the story is that in 1866, long after Adams had determined the orbit, a new comet was seen which was found to move exactly along the path of the November meteors, in the same way that the comet of 1812 agrees with those which fall in August.

Although these two meteor-showers are the most important, they are by no means the only ones crossed by our earth. Any clear night, if you watch carefully, you may see (according to the astronomer Proctor) about six shooting-stars in one hour; and Professor Newton, of America, has calculated that 7,500,000 meteors large enough to be seen without a telescope pass through our atmosphere in one single day and night. At least a hundred sets of meteors, or meteor-systems as they are called, are known to astronomers, and each one of these is composed of millions of bodies; and you must bear in mind that these systems do not move round us, but round the sun, so that it is only because we happen to cross their path that we know anything of them. It would be idle to

suppose that these hundred meteor-systems which we come across are the only ones existing. On the contrary, we have every reason to think that they are only a few out of thousands of meteor-systems which we never meet, and which must grow more numerous the nearer they approach the sun.

And so we arrive at the wonderful thought that the whole of our solar system is swarming with meteors rushing along with immense speed! What their use is we do not know. Some astronomers imagine that the heat of the sun is kept up by these meteoric stones falling in countless myriads on his face, but this is disputed by others; and for the present it is enough if we can picture to ourselves these rings of meteors whirling round and round in space, and flashing into light as they rush through our atmosphere whenever we happen to cross their path.

us.

I have chosen out these new facts about meteors because, of all modern discoveries, they give the best idea of the wide fields of knowledge which are opening out before Within the last fifty years a number of most interesting observations have been made about the nature of the sun itself; but they would require long explanations, and being all the work of living men they scarcely belong to our history.

In the chapter on Spectrum Analysis we shall learn something of the atmosphere of the sun and stars, and in the chapter on Magnetism something of the spots on the sun and their effects on our earth. But for the history of the discovery of the photosphere, corona, red prominences, and other wonderful appearances upon the face of the sun, you must read special works on the subject.

CH. XXXI.

ASTRONOMICAL WORKS.

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Chief Works consulted.-Airy's 'Report on Astronomy,' British Association, 1833; J. D. Forbes's Progress of Mathematical and Physical Science'-Sixth Dissertation; Encyclopædia Britannica,' new edition; Guillemin, The Heavens ; ' Herschel's 'Astronomy;' Grant's 'Physical Astronomy;' 'Reports of the Astronomical Society;' 'The Orbs of Heaven,' Mitchell; Proctor, 'On ShootingStars and Meteors

CHAPTER XXXII.

SCIENCE OF THE NINETEENTH CENTURY (CONTINUED). Discoveries concerning Light made in the Nineteenth Century-Birth and History of Dr. Young-He explains the Interference of Light -Cause of Prismatic Colours in a Shadow-And in a Soap-bubble -Malus discovers the Polarization of Light caused by ReflectionBirth and History of Fresnel-Polarization of Light explained by Young and Fresnel-Complex Vibrations of a Ray of Light-How these Waves are reduced to two separate Planes in passing through Iceland Spar-Sir David Brewster and M. Biot explain the colours produced by Polarization.

We must now go back to the history of Light, which we left, as you will remember, at the end of the seventeenth century, at the point which Newton and Huyghens had reached. During the whole of the eighteenth century very little was learnt about this science, and it remained for the men of our own time to make the next step and to discover the grand laws of light which we must now consider. It will be best to divide our subject into two parts-1st. The discoveries which have led to a true Theory of Light. These are very difficult to understand, and you must not expect to gain more than a slight notion of them; 2nd. The new facts lately discovered about the Chemistry of Light, and called Spectrum Analysis, and of these I hope you may understand enough to fill you with delight at the beautiful histories they reveal.

Discovery of the Interference of Light by Young, 1801.You will remember that Newton and Huyghens had proposed