386 THE POPULAR SCIENCE MONTHLY. 



surface of the earth ; when they do, they are called balls of fire ; and 

 occasionally they reach the earth before they are completely burnt out 

 or evaporated ; they are then termed meteoric stones, aerolites, or 

 meteoric iron. They are also divided into accidental meteors and 

 meteoric showers, according as to whether they traverse the heavens 

 in every direction at random, or appear in great numbers following a 

 common path, thus indicating that they are parts of a great whole. 



It is now generally received, and placed almost beyond doubt by 

 the recent observations of Schiaparelli, Le Verrier, Weiss, and others, 

 that these meteors, for the most part small, but weighing occasionally 

 many tons, are fragmentary masses, revolving, like the planets, round 

 the sun, which in their course approach the earth, and, drawn by its 

 attraction into our atmosphere, are set on fire by the heat generated 

 through the resistance offered by the compressed air. 



The chemical analysis of those meteors which have fallen to the 

 earth in a half-burnt condition in the form of meteoric stones proves 

 that they are composed only of terrestrial elements, which present a 

 form and combination commonly met with in our planet. Their chief 

 constitutent is metallic iron, mixed with various silicious compounds ; 

 in combination with iron, nickel is always found, and sometimes also 

 cobalt, copper, tin, and chromium ; among the silicates, olivine is 

 especially worthy of remark as a mineral very abundant in volcanic 

 rocks, as also augite. There have also been found, in the meteoric 

 stones hitherto examined, oxygen, hydrogen, sulphur, phosphorus, 

 carbon, aluminium, magnesium, calcium, sodium, potassium, manganese, 

 titanium, lead, lithium, and strontium. 



The height at which meteors appear is very various, and ranges 

 chiefly between the limits of 46 and 92 miles ; the mean may be taken 

 at 66 miles. The speed at which they travel is also various, generally 

 about half as fast again as that of the earth's motion round the sun, or 

 about 26 miles in a second : the maximum and minimum differ greatly 

 from this amount, the velocity of some meteors being estimated at 14 

 miles, and that of others at 107 miles in a second. 



When a dark meteorite of this kind, having a velocity of 1,660 

 miles per minute, encounters the earth, flying through space at a mean 

 rate of 1,140 miles per minute, and when through the earth's attraction 

 its velocity is further increased 2.30 miles per minute, this body meets 

 with such a degree of resistance, even in the highest and most rarefied 

 state of our atmosphere, that it is impeded in its course, and loses in a 

 very short time a considerable part of its momentum. By this 

 encounter there follows a result common to all bodies which, while in 

 motion, suddenly experience a check When a wheel revolves very 

 rapidly, the axletree or the drag which is placed under the wheel is 

 made red-hot by the friction. When a cannon-ball strikes suddenly 

 with great velocity against a plate of iron, which constantly happens 

 at target-practice, a spark is seen to flash from the ball even in day- 



