128 Mr. N. S. Maskelyne on the Pitted 



He showed that by simply removing the gases rapidly 

 enough, as in an exhausted vessel, you might arrest entirely 

 the combustion of gunpowder — and, further, that another 

 element in the rapidity with which the combustion penetrates a 

 solid mass of that explosive is the density of the mass. The 

 huge cubes of nearly two inches, no less than the other 

 varieties of " pebble " and " prismatic " forms of coarse-grain 

 powder employed in our large guns are, by their very manu- 

 facture, lacking in mechanical homogeneity. 



Within the gun, where the pressure is some twenty-two or 

 twenty-three tons to the square inch, the process of combus- 

 tion goes on rapidly, but can be regulated by adjusting the 

 density ; and size of the gas-producing grains of gunpowder. 

 It furthermore goes on with greater intensity at some than at 

 other parts of the surface, in consequence of the variations in 

 density ; and at these points centres of combustion become 

 established — the relative depths in different parts of the sur- 

 face to which the combustion would penetrate in any brief 

 period depending on the relative densities of the ingredient 

 granules or fragments of " cake," which give to the gun- 

 powder mass the character of a sort of breccia. At the dis- 

 charge of the gun the released gases escape, together with a 

 few fragments, which are the cores of some of the cubes of 

 gunpowder which have not had time to be entirely consumed ; 

 and at the surface of these fragments the combustion will 

 proceed at different centres of combustion, the depth of which 

 depends on the density of the powder and the pressure exerted 

 on it by the surrounding gases. 



At the same time the sudden removal of the vast pressure 

 in operation within the gun, and an initial velocity of 1400 feet 

 in a second, sufficiently explain, on Professor Abel's principle, 

 the extinction of the combustion of these projected fragments. 

 Their pitted surfaces are also explained as the result of me- 

 chanical heterogeneity in the component particles of the 

 powder. 



With these points M. Daubree appears to be in accord, 

 though he does not seem to have been aware of Professor 

 Abel's researches, as he does not refer to them. 



Let us now turn to the case of a meteorite. It enters our 

 atmosphere with at least a planetary velocity, which is at once 

 arrested by the resistance the body experiences in traversing 

 even the highest and rarest region of the aerial envelope of 

 our globe. 



The resulting heat at once fuses the surface of the stone or 

 iron; and though the fused and oxidized material is thrown off 

 (aided in most cases by a rotatory movement in the mass) as 



