7 6 THE POPULAR SCIENCE MONTHLY. 



since it is found coating surfaces of fragments that have been severed 

 by the explosions in the air. 



Aerolites frequently fall simultaneously in large numbers, many 

 thousands of them being in such cases spread over a surface of the 

 country some miles in extent ; and such showers of stones seem to have 

 entered the atmosphere as a group, though their numbers must sub- 

 sequently have been greatly increased by the division accompanying 

 their detonation. 



The explanation of the incrustation and of the cloud left by the 

 meteorite, or out of which it seems to emerge, is found in the trans- 

 formation into heat of the energy actuating a body that enters our 

 atmosphere with a motion of 12 to 40 miles a second. The velocity 

 of the body is almost instantaneously arrested by the atmospheric 

 resistance, and in a very few seconds the mass becomes, comparatively 

 speaking, stationary. Its surface must, as a consequence, be imme- 

 diately fused, and the melted matter would be flung off from it into 

 the surrounding air, fresh surfaces continually affording new fused 

 material to form the cloud of, so to say, siliceous spray that lingers 

 along and around the path of the meteorite. 



When the mass is small and in the case of meteoric showers and 

 ordinary falling stars it cannot exceed a few ounces, and may often 

 be but a few grains the whole material is thus consumed, and must 

 ultimately fall as an unperceived, because widely-scattered, dust. The 

 meteorite is the residue that survives this wastinsr action where the 

 magnitude of the mass is more considerable. The cause of the violent 

 and often successive explosions is probably to be sought in the expan- 

 sion of the outer portions of the mass, while the interior retains the 

 contracted volume due to the intense cold of space with which the 

 meteorite enters the atmosphere. 



From time to time these contending conditions of volume may, as 

 in a Prince Rupert's drop, produce explosion, the heated shell in the 

 case of the meteorite flying off in fragments from the internally cold 

 inner core, which if sufficient velocity remain to the mass will undergo 

 a recurrence of the same conditions of surface fusion and explosion. 

 The loudness of the detonation is also probably enhanced by the 

 simultaneous collapse of the air on the vacuum that would follow the 

 rapidly-moving mass. 



The pitted surface characteristic of meteorites probably bears 

 witness to a similar effect of unequal dilatation operating more espe- 

 cially in the freshly-broken surfaces of the mass, small fragments 

 splintering off in this way from the cold and brittle stone under the 

 sudden influence of intense heat. 



A remark made by Humboldt, that light and meteorites are the 

 only sources of our knowledge regarding the universe external to our 

 world, points to the true ground for our interest in the waifs and strays 

 of extra-telluric matter that thus fall upon our globe. 



