METEORIC STONES AND SHOOTING STARS. 



431 



masses thus precipitated, several circumstances worthy to be noted are pre- 

 sented. It is found that their surfaces are generally black, having a burnt ap- 

 pearance ; but the most remarkable circumstance attending them is, that at what- 

 ever time, or in whatever part of the earth they may have fallen, they generally 

 consist of the same constituent parts, and always very nearly in the same pro- 

 portion. Their ingredients are silex, magnesia, sulphur, iron, nickel, and chro- 

 mium. There is occasionally, but not invariably, a trace of charcoal. 



It is important to observe here, that the iron and nickel found in these bodies 

 are always in the metallic form a state in which they are never known to ex- 

 ist naturally on the surface of the earth. These metals, when found in the 

 earth, are invariably combined with oxygen, and it is their oxides only which 

 have a place among natural terrestrial substances. The iron and nickel used 

 in the arts are obtaiaed by the decomposition of the ores in the process of met- 

 allurgy. 



The distances from the earth at which these meteors pass when they are 

 visible has been ascertained with a tolerable degree of approximation, by ob- 

 serving the length and position of their visible course at the same time from two 

 distant places. It has been found by these means that they are frequently visi- 

 ble at the height of from 30 to 40 miles. This is generally considered as the 

 limit of the height of the atmosphere. 



Such are the circumstances attending the exhibition of these meteors, which 

 have been collected from careful and accurate information. Let us now turn 

 our attention to the different methods by which it has been attempted to explain 

 them. Three different hypotheses, or theories, have been proposed for this ) 

 purpose. 



First. It is supposed that the matter composing them has been drawn up 

 from the surface of the earth in a state of infinitely minute subdivision, as va- 

 por is drawn from liquids ; that, being collected in clouds in the higher regions 

 of the atmosphere, it is there agglomerated and consolidated in masses, and 

 falls by its gravity to the surface of the earth ; being occasionally drawn from 

 the vertical direction which would be imparted to it by gravity, by the effect of 

 atmospheric currents, and thus occasionally striking the earth obliquely. We 

 shall call this the atmospheric hypothesis. 



Secondly. It is supposed that meteoric stones are ejected from volcanoes, 

 with sufficient force to carry them to great elevations in the atmosphere, in 

 falling from which they acquire the velocity and force with which they strike 

 the earth. The oblique direction with which they strike the ground is ex- 

 plained by the supposition that they may be projected from the volcanoes at 

 corresponding obliquities, and that, by the principles of projectiles, they must 

 strike the earth at nearly the same inclination as that with which they have 

 been ejected. This we shall call the volcanic hypothesis. 



Thirdly. It has been supposed that these bodies are not either terrestrial or 

 atmospheric, but belonging to the solar system ; and that their origin is the 

 same as that which has produced the small planets which have been discovered 

 moving between the orbits of Mars and Jupiter. 



This theory supposes that, at some former epoch, the solar system possessed 

 a planet which revolved round the sun at the distance of two hundred and fifty 

 millions of miles ; a supposition which is rendered highly probable, if not mor- 

 ally certain, by reasons which are fully detailed in my discourse on the new 

 planets. The catastrophe by which this former planet was broken into pieces 

 is supposed to have been produced, either by internal explosion (from some 

 cause similar to that which produces on the earth volcanoes and earthquakes), 

 or by the collision of a comet. It is supposed that the new planets are not the 

 only fragments which resulted, but that innumerable smaller pieces may have 



