278 METEORS AND AEROLITES. 



from some larger mass. Their specific gravity varies greatly 

 according to the proportion of metallic constituents, ranging 

 from twice even to six or seven times the weight of water. 

 The mean is considerably above that of the mineral masses 

 on the surface of the earth, though much below 5*5, or the 

 mean of the whole earth. A notable and very uniform cha- 

 racter of aerolites is the shining dark crust enveloping them. 

 It is generally very thin ; but indicates by its aspect, and by 

 its entire separation from the mass within, some rapid action 

 of heat, which has not had time to penetrate more deeply into 

 the substance of the stone. These appearances may give useful 

 guidance to any local search for aerolites of ancient record. 



The question as to the mean velocity of aerolites in ap- 

 proaching the earth, can only be settled by approximation, 

 and that perhaps not a very close one. The observations 

 bearing on this point are limited in great degree to the 

 meteoric appearances preceding the fall. The conclusions 

 obtained by Olbers and others would justify the belief in a 

 mean velocity exceeding twenty miles "in a second ; a rate 

 of movement further attested by the depth to which many 

 of them penetrate into the earth ; and becoming, as we shall 

 presently see, an important element towards the solution of 

 many questions in the theory of these bodies. 



The main facts as to Aerolites thus authenticated, the 

 question as to their origin comes yet more forcibly into view. 

 And, in truth, there are few questions more curious - not 

 less to the unenlightened than to men of science in the 

 novelty and vastness of the suggestions they press upon the 

 mind. Whence, and by what force, do these stones some 

 of them so massive, all so remarkable in composition 

 descend upon the earth? 



It could scarcely perhaps be surmised that five different 

 solutions have been offered in answer to this question. We 

 might even name six, could we for a moment admit the 



