The Story of the Heavens, Sir Robert Stawell Ball [snow like ashes series TXT] 📗

pure iron. We know the circumstances in which that piece of iron fell to the earth. It was on the 20th of April, 1876, about 3.40 p.m., that a strange rumbling noise, followed by a startling explosion, was heard over an area of several miles in extent among the villages in Shropshire, eight or ten miles north of the Wrekin. About an hour after this occurrence a farmer noticed that the ground in one of his grass-fields had been disturbed, and he probed the hole which the meteorite had made, and found it, still warm, about eighteen inches below the surface. Some men working at no great distance had heard the noise made in its descent. This remarkable object, weighs 7-3/4 lbs. It is an irregular angular mass of iron, though all its edges seem to have been rounded by fusion in its transit through the air. It is covered with a thick black pellicle of the magnetic oxide of iron, except at the point where it first struck the ground. The Duke of Cleveland, on whose property it fell, afterwards presented it to our national institution already referred to, where, as the Rowton siderite, it attracts the attention of everyone who is interested in these wonderful bodies.

This siderite is specially interesting on account of its distinctly metallic character. Falls of objects of this particular type are not so frequent as are those of the stony meteorites; in fact, there are only a few known instances of meteoric irons having been actually seen to fall, while the observed falls of stony meteorites are to be counted in scores or in hundreds. The inference is that the iron meteorites are much less frequent than the stony ones. This is, however, not the impression that the visitor to the Museum would be likely to receive. In that extensive collection the meteoric irons are by far the most striking objects. The explanation is not difficult. Those gigantic masses of iron are unquestionably meteoric: no one doubts that this is the case. Yet the vast majority of them have never been seen to fall; they have simply been found, in circumstances which point unmistakably to their meteoric nature. Suppose, for instance, that a traveller on one of the plains of Siberia or of Central America finds a mass of metallic iron lying on the surface of the ground, what explanation can be rendered of such an occurrence? No one has brought the iron there, and there is no iron within hundreds of miles. Man never fashioned that object, and the iron is found to be alloyed with nickel in a manner that is always observed in known meteorites, and is generally regarded as a sure indication of a meteoric origin. Observe also, that as iron perishes by corrosion in our atmosphere, that great mass of iron cannot have lain where it is for indefinite ages; it must have been placed there at some finite time. Only one source for such an object is conceivable; it must have fallen from the sky. On the same plains the stony meteorites have also fallen in hundreds and in thousands, but they crumble away in the course of time, and in any case would not arrest the attention of the traveller as the irons are likely to do. Hence it follows, that although the stony meteorites seem to fall much more frequently, yet, unless they are actually observed at the moment of descent, they are much more liable to be overlooked than the meteoric irons. Hence it is that the more prominent objects of the British collection are the meteoric irons.

We have said that a noise accompanied the descent of the Rowton siderite, and it is on record that a loud explosion took place when the meteorite fell at Ensisheim. In this we have a characteristic feature of the phenomenon. Nearly all the descents of meteorites that have been observed seem to have been ushered in by a detonation. We do not, however, assert that this is quite an invariable feature; and it is also the case that meteors often detonate without throwing down any solid fragments that have been collected. The violence associated with the phenomenon is forcibly illustrated by the Butsura meteorite. This object fell in India in 1861. A loud explosion was heard, several fragments of stone were collected from distances three or four miles apart; and when brought together, they were found to fit, so as to enable the primitive form of the meteorite to be reconstructed. A few of the pieces are wanting (they were, no doubt, lost by falling unobserved into localities from which they could not be recovered), but we have obtained pieces quite numerous enough to permit us to form a good idea of the irregular shape of the object before the explosion occurred which shattered it into fragments. This is one of the ordinary stony meteorites, and is thus contrasted with the Rowton siderite which we have just been considering. There are also other types of meteorites. The Breitenbach iron, as it is called, is a good representative of a class of these bodies which lie intermediate between the meteoric irons and the stones. It consists of a coarsely cellular mass of iron, the cavities being filled with mineral substances. In the Museum, sections of intermediate forms are shown in which this structure is exhibited.

Look first at the most obvious characteristic of these meteorites. We do not now allude to their chemical composition, but to their external appearance. What is the most remarkable feature in the shape of these objects?--surely it is that they are fragments. They are evidently pieces that are _broken_ from some larger object. This is apparent by merely looking at their form; it is still more manifest when we examine their mechanical structure. It is often found that meteorites are themselves composed of smaller fragments. Such a structure may be illustrated by a section of an aerolite found on the Sierra of Chaco, weighing about 30 lbs. (Fig. 79).

The section here represented shows the composite structure of this object, which belongs to the class of stony meteorites. Its shape shows that it was really a fragment with angular edges and corners. No doubt it may have been much more considerable when it first dashed into the atmosphere. The angular edges now seen on the exterior may be due to an explosion which then occurred; but this will not account for the structure of the interior. We there see irregular pieces of varied form and material agglomerated into a single mass. If we would seek for analogous objects on the earth, we must look to some of the volcanic rocks, where we have multitudes of irregular angular fragments cemented together by a matrix in which they are imbedded. The evidence presented by this meteorite is conclusive as to one circumstance with regard to the origin of these objects. They must have come as fragments, from some body of considerable, if not of vast, dimensions. In this meteorite there are numerous small grains of iron mingled with mineral substances. The iron in many meteorites has, indeed, characters resembling those produced by the actual blasting of iron by dynamite. Thus, a large meteoric iron from Brazil has been found to have been actually shivered into fragments at some time anterior to its fall on the earth. These fragments have been cemented together again by irregular veins of mineral substances.

For an aerolite of a very different type we may refer to the carbonaceous meteorite of Orgueil, which fell in France on the 14th May, 1864. On the occasion of its descent a splendid meteor was seen, rivalling the full moon in size. The actual diameter of this globe of fire must have been some hundreds of yards. Nearly a hundred fragments of the body were found scattered over a tract of country fifteen miles long. This object is of particular interest, inasmuch as it belongs to a rare group of aerolites, from which metallic iron is absent. It contains many of the same minerals which are met with in other meteorites, but in these fragments they are _associated with carbon_, and with substances of a white or yellowish crystallisable material, soluble in ether, and resembling some of the hydrocarbons. Such a substance, if it had not been seen falling to the earth, would probably be deemed a product resulting from animal or vegetable life!

We have pointed out how a body moving with great velocity and impinging upon the air may become red-hot and white-hot, or even be driven off into vapour. How, then, does it happen that meteorites escape this fiery ordeal, and fall down to the earth, with a great velocity, no doubt, but still, with very much less than that which would have sufficed to drive them off into vapour? Had the Rowton siderite, for instance, struck our atmosphere with a velocity of twenty miles a second, it seems unquestionable that it would have been dissipated by heat, though, no doubt, the particles would ultimately coalesce so as to descend slowly to the earth in microscopic beads of iron. How has the meteorite escaped this fate? It must be remembered that our earth is also moving with a velocity of about eighteen miles per second, and that the _relative_ velocity with which the meteorite plunges into the air is that which will determine the degree to which friction is operating. If the meteorite come into direct collision with the earth, the velocity of the collision will be extremely great; but it may happen that though the actual velocities of the two bodies are both enormous, yet the relative velocity may be comparatively small. This is, at all events, one conceivable explanation of the arrival of a meteorite on the surface of the earth.

We have shown in the earlier parts of the chapter that the well-known star showers are intimately connected with comets. In fact, each star shower revolves in the path pursued by a comet, and the shooting star particles have, in all probability, been themselves derived from the comet. Showers of shooting stars have, therefore, an intimate connection with comets, but it is doubtful whether meteorites have any connection with comets. It has already been remarked that meteorites have never been known to fall in the great star showers. No particle of a meteorite is known to have dropped from the countless host of the Leonids or of the Perseids; as far as we know, the Lyrids never dropped a meteorite, nor did the Quadrantids, the Geminids, or the many other showers with which every astronomer is familiar. There is no reason to connect meteorites with these showers, and it is, therefore, doubtful whether we should connect meteorites with comets.

With reference to the origin of meteorites it is difficult to speak with any great degree of confidence. Every theory of meteorites presents difficulties, so it seems that the only course open to us is to choose that view of their origin which seems least improbable. It appears to me that this condition is fulfilled in the theory entertained by the Austrian mineralogist, Tschermak. He has made a study of the meteorites in the rich collection at Vienna, and he has come to the conclusion that the "meteorites have had a volcanic source on some celestial body." Let us attempt to pursue this reasoning and discuss the problem, which may be thus stated:--Assuming that at least some of the meteorites have been ejected from volcanoes, on what body or bodies in the universe must these volcanoes be situated? This is really a question for astronomers and mathematicians. Once the mineralogists assure us that these bodies are volcanic, the question becomes one of calculation and of the balance of probabilities.

The first step in the enquiry is to realise distinctly the dynamical conditions of the problem. Conceive a volcano to be