1874.] 1.^7 [Marsh. 



heat to distribute itself through the body of the meteor, the whole of its 

 effect is confined to the surface ; extremely thia layers of which are, in 

 succession, heated, rendered intensely incandescent, and vapoiized, how- 

 ever refractory the material. 



The black "crust," of the thickness of letter paper, with which the 

 stony meteorites are coated, shows the limits for any one instant, of the 

 melting process ; and the fact, that beneath the crust there is no trace of 

 the action of fire, is proof, both of the extreme intensity of the heat, and 

 of its entirely superficial distribution. 



Another disintegrating process may, perhaps, be mainly confined to the 

 smaller meteorites and to the ordinary shooting stars, which are so com- 

 pletely. dissipated that no trace of them reaches the earth. Ahhough, in 

 any individual layer, the three states — solid, liquid, and vapor — exist al- 

 most at the same instant, they must in reality succeed each other in the 

 order named ; so that there must always be a layer in which the material, 

 although not melted, is so intensely heated as to exert an expansive 

 energy tending to split the mass into Iragments. The amount of de- 

 crepitation thus produced must, of course, depend upon the brittleness 

 and other peculiarities of the meteor, as well as ui)on its velocity and 

 upon the density of the air encountered ; but the efl^ect must be similar 

 in character to that which takes place when coal being thrown upon the 

 fire of a locomotive, minute fragments split off by the sudden expansion, 

 are carried up the chimney and fall upon the car-roof in such numbers 

 as to reaiind passengers of the rattle of a shower of hail. 



It can scarcely be supposed that combu'<tion has much to do with the 

 splendor of meteors, or with th^ir destruction, since these mainly occur 

 at heights at which there is not air enough to maintain combustion to 

 any considerable extent. Their disintegration must therefore be mainly 

 effected by heat alone, unaided by chemical action. 



Frequently, after the disappearance of a meteor of extraordinary 

 splendor, a luminous train or cloud remains for a few seconds, sometimes 

 for several minutes, and in some very rare instances they have remained 

 visible for an hour or more. A remarkable example of this occurred on 

 the 14th of November, 1868, when, shortly after midnight, a meteor ap- 

 pearing over Kortheasterh Pennsylvania, left a cloud which remained 

 visible to observers at Washington and New Haven and at all interme- 

 diate points, for about three-quarters of an hour. According to Prof. 

 Newton,* the observations indicatpd for this cloud "a real diameter of 

 one mile, and a volume of a dozen or a score of cubic miles," and that 

 whilst visible it moved about forty miles, showing an average velocity 

 relatively to the earth of nearly a mile per minute. What was its velocity 

 relatively to the air is not known. This cloud was, no doubt, the debris 

 of the meteor, a cloud of meteoi'ic dust, moving rapidly through the air, 

 compressing the air before it ; and, .of course, if the above views be cor- 

 rect, developing heat and light, just as, on a grander scale, heat and light 



* Silliman's Journal, vol. 47, p. 406. 



