Outlines of Meteoric Astronomy. 215 



(in the Old and New World), present the average values of 

 62 and 35 miles. The upper limit appears therefore suffi- 

 ciently uncertain ; but the lower limit is more depressed in the 

 air than in the case of shooting-stars. Fire-balls which pene- 

 trate to within twenty miles of the earth's surface, generally 

 produce violent concussions in the air, and are called Detonat- 

 ing fire-balls. These frequently precipitate stones upon the 

 ground, and are occasionally called Aerolitic meteors, to distin- 

 guish them from the class of silent fire-balls, or Bolides. 

 Bolides vary in magnitude from the apparent diameters of the 

 planets to fire-balls which throw shadows, and are often bril- 

 liant and surprising objects at noonday. The velocities of 

 bolides, as well as of detonating fire-balls, average more than 

 thirty miles per second, and no distinction can be drawn in this 

 respect between shooting- stars and fire-balls. 



(10.) The coincidence of dates and directions evince 

 the common origin of bolides and periodical shooting- stars. 

 Only those fire-balls which are truly aerolitic will, it ap- 

 pears, require to be explained in a different manner. Among the 

 240,000 meteors said to have been visible above the horizon in 

 America, during the night of the 12th — -13th November, 1833, a 

 considerable proportion consisted of fire-balls, varying in bright- 

 ness from the apparent magnitudes of Jupiter and Venus to 

 that of the Moon. The same was also observed in England, 

 though on a less striking scale, on ,the night of the 10th of 

 August, 1863. Shooting-stars and small bolides radiated on 

 that night from ten o'clock till midnight, from the direction of 

 the usual radiant point in Perseus. Again, a catalogue of 

 nearly 1500 fire-balls, compiled from all previous catalogues, 

 and from other independent sources, by Mr. Greg, shows that a 

 large preponderance of fire-balls were observed on the dates 

 of the 1 0th of August, 13th of November, and 6th — 12th 

 of December. Aerolites, on the contrary, are wanting on the 

 same dates ; for if the aerolitic date on the 13th — 1.4th of De- 

 cember may be omitted, there is no approximation of aerolites 

 to the star-showers of August and November. Fire-balls are 

 shown by the catalogue to be abundant on the 10th of April 

 and 18th of October, and on both of these dates star-showers 

 have been observed. There appears therefore a variety of 

 evidence to show that shooting- stars and fire-balls Lave a 

 common origin ; and furthermore that the sun is irregularly 

 encompassed by zones of meteoric dust, concentrated at some 

 points into shooting-stars and bolides, which by some unknown 

 predisposing cause have been collected into star-showers. The 

 concentration of the materials appears to have taken place with- 

 out consolidation, nevertheless some portions might be so 

 far compacted by chemical affinities or crystallization, as to 



