THE ORIGIN AND STRUCTURE OF METEORITES. 377 



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 specta- 

 tors 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 con- 

 trast 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, en- 

 tirely 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 atmos- 

 phere. 



It is, unfortunately, very rarely possible to find the fragments pro- 

 jected by meteors ; and it is only under quite exceptional circum- 

 stances, even in populous countries, that one is discovered among the 

 clods and under the vegetation by which they are commonly con- 

 cealed. 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 vol- 

 canoes in the moon. If this were the case, they would have to be sup- 

 posed to have been ejected by the eruption with velocity enough to 

 pass the neutral point, or the point where a body is equally attracted 



