378 THE POPULAR SCIENCE MONTHLY, 



by the moon and the earth. That velocity should be at least 2,270 

 metres a second, or about five times that of a cannon-ball ; if it were 

 less, the mass would fall back to the moon. Another more probable 

 supposition is that they come from a group of minute asteroids which 

 revolve in the space between Mars and Jupiter, whose orbits cross those 

 of the large planets, ours included, and are occasionally met by the 

 earth in its course. There is nothing else, since the beautiful re- 

 searches of Schiaparelli have connected the periodical swarms of shoot- 

 ing-stars with comets, to assure us that they do not come from still 

 more distant parts of the sky, or even from without the solar system. 



Shooting-stars come to us by millions at regular periods ; and the 

 number of those which are directed toward our globe in a single year 

 is estimated at many milliards. They somewhat resemble meteorites in 

 the abruptness of their appearance in our atmosphere and the exces- 

 sive rapidity of their motion, but they differ from them in an impor- 

 tant characteristic. None of them ever reach the ground. They ap- 

 pear to share in the properties of comets, from which they may have 

 been dismembered and told off by perturbing actions ; while meteor- 

 ites seem to be related to the planets. The difference between them 

 is like the difference between gases or vapors and solid bodies. 



The meteors coming to our earth without, excepting as to their 

 superficial vitrification, undergoing any change, we are able, by subject- 

 ing them to analysis, to derive from them some precise facts respect- 

 ing the constitution of the bodies in space. The first fact, which 

 comes out from hundreds of analyses, is, that they have not brought 

 a single substance which is foreign to our globe. About twenty-two 

 elements, all known to the chemistry of the earth, have been recog- 

 nized as present in them. Among these, iron, silicon, magnesium, 

 nickel, sulphur, phosphorus, and carbon, are the most important. 

 While they are all clad externally in a common livery, meteorites, 

 when examined in their fractured parts, along with traits of similarity, 

 present considerable differences. They have been classified, according 

 to their types, into four groups, according to the proportion of iron 

 they contained. Those of the first group are composed almost wholly 

 of iron, which is known as meteoric iron. It is always alloyed with 

 nickel and a few other metals, and contains carbon free or in combina- 

 tion, as in steel, with frequently sulphuret and phosphuret of iron in 

 scattered globules and grains. It is always recognizable by a single 

 peculiarity in its structure. If we moisten a polished surface of it 

 with an acid, we shall immediately observe the appearance of numer- 

 ous straight lines, as fine and as true in their parallelism as if made with 

 an engraver's tool, and crossing one another in a net-work of regular 

 geometrical figures. These designs, called the figures of Widman- 

 staetten, after the first observer of them, result from the fact that the 

 metal is not of homogeneous constitution. It is composed of two 

 alloys of iron and nickel, in a crystalline condition, one of which, not 



