Page:Popular Science Monthly Volume 29.djvu/391

Rh axis of which corresponds with the direction of the trajectory, and within which they appear to have been sifted by the resistance offered by the atmosphere, in the order of their magnitude.

Meteors are not incandescent when they reach the ground, but are still too hot to handle. Sometimes the high temperature is limited to their surface, while within they are extraordinarily cold. The spectators of a meteoric fall at Dhurmsalla, India, on the 14th of July, 1860, eagerly broke up the stones, still burning hot on the outside, and were greatly surprised to find that it was impossible to handle the inside parts on account of their extreme coldness. A similar observation was made on the 16th of May, 1883, at Alfianello, near Brescia. This contrast between the central part, still retaining the intense cold of the planetary spaces, and the outside, which only a few moments before had been red-hot, may be easily understood when we reflect on the feebleness of the conducting powers of stony substances, and the very short time that they had been heated.

One effect of this heat persists, and is obvious at first sight as a general characteristic of meteorites, in the shape of a black crust, entirely covering them. It is not a millimetre thick, and is generally dead, but forms in some especially fusible types a glossy enamel. The same effect, of vitrification, is produced by lightning on rocks which are struck by it. The incandescence of which this is the effect, and which had been observed in the meteor flying in the distance, is the result of the extreme speed with which the body penetrates the atmosphere.

It is, unfortunately, very rarely possible to find the fragments projected by meteors; and it is only under quite exceptional circumstances, even in populous countries, that one is discovered among the clods and under the vegetation by which they are commonly concealed. The observer enjoys the illusion of supposing he sees them fall at no great distance from him; but he will hardly ever find one if he looks for it. Probably three quarters of them are swallowed up by the sea.

Supposing there are three meteoric showers a year in Europe—and this is the mean of what has been observed there—and that that part of the earth is not exceptionally favored by them, we have one hundred and eighty a year for the whole surface of the globe. But, as many of the showers are not perceived, we may safely triple the figure, or even suppose there are six hundred, and still underestimate the reality. We are dealing, therefore, with a daily phenomenon.

We do not know in what regions of space meteors originate, nor what courses they follow before they come within the sphere of the earth's attraction. They have been supposed to be ejections from volcanoes in the moon. If this were the case, they would have to be supposed to have been ejected by the eruption with velocity enough to pass the neutral point, or the point where a body is equally attracted