siderably earlier. Had the actual number of meteors encountered by the Earth remained the same the apparent number would have increased from midnight to 6 A.M.; as at 6 we should have been nearly in the middle of the front side of the Earth on which they would be showering.

308. By careful observations of the radiant-point it has been determined that the orbit of each member of the November star-shower, and therefore of the whole mass, is an ellipse with its perihelion lying on the Earth's orbit, and its aphelion point lying just beyond the orbit of Uranus; that its inclination to the plane of the ecliptic is 17°; and that the direction of the motion of the meteors is retrograde.

309. Up to the present time several hundred such radiant-points, which possibly indicate several hundred other similar groups moving round the Sun in cometary or planetary orbits, have been determined. The meteors of particular showers vary in their distinctive characters, some being larger and brighter than others, some whiter, some more ruddy than others, some swifter, and drawing after them more persistent trains than those of other showers.

LESSON XXV.-Luminous Meteors (continued). Cause of the Phenomena of Meteors. Orbits of Shooting Stars. Detonating Meteors. Meteorites: their Classification. Falls. Chemical and Physical Constitution.

310. Now let us take the case of a single meteor entering our atmosphere. Why do we get such a brilliant appearance? In the first place, we have the Earth travelling at the rate of 1,000 miles a minute, plunging into a mass of bodies whose velocity is at first equal to its own, and is then increased to 1,200 miles a minute by the Earth's

attraction. The particle then enters our atmosphere at the rate of 30 miles a second; its motion is arrested by the friction of that atmosphere, which puts a break on it, and as the wheel of a tender gets hot under the same circumstances, and as a cannon-ball gets hot when the target impedes its further flight, so does the meteoric particle get hot. So hot does it get that, at last, as great heat is always accompanied by light, we see it it becomes vaporized, and leaves a train of luminous vapour behind it.

311. It would seem that all the particles which compose the November shower are small: it has been estimated that some of them weigh but two grains. They begin to burn at a height of 74 miles, and are burnt up and disappear at a height of 54 miles; the average length of their visible paths being 42 miles. It is supposed that the November-shower meteors are composed of more easily destructible or of more inflammable materials than aërolitic bodies.

312. What has been said about the appearance of the November meteors applies to the other star-showers, notably to the August and April ones, the meteors of which also travel round the Sun in cometary orbits. And this brings us to one of the most surprising discoveries of modern times—a discovery for which our consideration of the fate of Biela's comet has in some degree prepared us. It has been ascertained that four well-known comets follow each an identical track with a meteor-stream; and the number of probable or suspected similar coincidences now amounts to 76. The August meteors (called "Perseids" from the situation of their radiant-point in the constellation Perseus) were the first to have a comet-the bright one of 1862-thus associated with them. Temple's comet of 1866, the comet 1861 I., and Biela's were immediately afterwards found to pursue tracks undistinguishable respectively from those belonging to the "Leonids" (November meteors), the "Lyraids" (April meteors), and the "Andromedes"

(Biela meteors). We must therefore come to the conclusion that the meteor-swarms composing comets are nothing more than denser aggregations in streams of meteorites which revolve in orbits round the sun.

313. In the case of the November and August meteors and shooting-stars generally, the mass is so small that it is entirely changed into vapour and disappears without noise. There are other classes of meteoric bodies, however, with much more striking effects. At times meteors of great brilliancy are heard to explode with great noise;

FIG. 31.-Fire-ball, as observed in a telescope.

these are called detonating meteors. On Nov. 15, 1859, a meteor of this class passed over New Jersey; it was visible in the full sunlight, and was followed by a series of terrific explosions, which were compared to the discharge of a thousand cannons.* Other meteors are so large that they reach the Earth before complete vaporization takes place, and we then get what is called a fall of meteoric irons, or meteoric stones, often accompanied by loud explosions.

314. Meteorites is the name given to those masses which, owing to their size, resist the action of the atmosphere, and actually complete their fall to the Earth. They are divided into aërolites, or meteoric stones;

* Professor Loomis.

aërosiderites, or meteoric iron; and aërosiderolites, which includes the intervening varieties.

315. We do not know whether these meteors which occasionally appear, and which are therefore called sporadic meteors-a term which includes meteors commonly so called, bolides, stonefalls, and ironfalls—belong to groups cometic or otherwise, although, like the falling stars, they affect particular dates; but, as they are independent of geographical position, it has been imagined that there may be some astronomical and perhaps a physical difference between them and the ordinary falling or shooting stars.

316. Among the largest aërolitic falls of modern times we may mention the following. On April 26th, 1803, at 2 P.M. a violent explosion was heard at L'Aigle (in Normandy); and at a distance of eighty miles round, a few minutes before the explosion was heard, a luminous meteor with a very rapid motion appeared in the air. Two thousand stones fell, so hot as to burn the hands when touched, and one person was wounded by a stone upon the arm. The shower extended over an area nine miles long and six miles wide, close to one extremity of which the largest of the stones was found. A similar shower of stones fell at Stannem, between Vienna and Prague, on the 22nd of May, 1812, when 200 stones fell upon an area eight miles long by four miles wide. The largest stones in this case were found, as before, near the northern extremity of the ellipse. A third stonefall occurred at Orgueil, in the south of France, on the evening of the 14th of May, 1864. The area in which the stones were scattered was eighteen miles long by five miles wide, and the luminous effects of their fall were visible from Paris to the Pyrenees. At Knyahinya, in Hungary, on the 9th of June, 1866, a luminous meteor was seen, and an aërolite weighing six hundredweight, and nearly one thousand lesser stones, fell on an area measuring ten miles in length by four miles wide. The large mass was found, as in the other cases, at one

extremity of the oval area; the fall was followed by a loud explosion, and a smoky streak was visible in the sky for nearly half an hour.*

317. A chemical examination of these fragments (a magnificent collection of which is to be seen in the British Museum) shows that, although in their composition they are unlike any other natural product, their elements are all known to us, and that they are all built up of the same materials, although in each variety some particular element may predominate. In the main, they are composed of metallic iron and various compounds of silica, the iron forming as much as 95 per cent. in some cases, and only I per cent. in others; hence the three classifications referred to in Art. 314. The iron is always associated with a certain quantity of nickel, and sometimes with manganese cobalt, copper, tin, and chromium. Among the silicates may be mentioned olivine, a mineral found abundantly in volcanic rocks, and augite.

318. Besides these substances, a compound of iron, phosphorus, and nickel, called schreibersite, is generally found: this compound is unknown in terrestrial chemistry. Carbon has also been detected.

319. The substances found in meteorites up to the present time are as follows :—

Aërosiderites.-Nickel-iron, copper, manganese. Troilite = Ferrous sulphide. Graphite. Schreibersite iron and nickel phosphide. Daubréeite - = iron and chromium sulphide.

Aërosiderolites. (a) Non-carbonaceous silicate of magnesium and iron. Enstatite

. Olivine =

= Silicate of

magnesium. Nickel-iron, manganese. Troilite. Chromite iron and chromium oxides. Augite silicate of

* Professor Herschel.

=