4 
PROFESSOIl G. H. DARWIN ON THE MECHANICAL CONDITIONS OF 
In a direct collision each stone is probably shattered into fragments, like the 
splashes of lead when a bullet hits an iron target. But direct collision must be 
a comparatively rare event. In glancing collisions the velocity of neither body is 
wholly arrested, the concentration of energy is not so enormous (although probably 
still sufficient to effect volatilisation), and, since the stones rub past one another, more 
time is allowed for the matter round the point of contact to take up the energy; thus, 
the whole process of collision is much more intelligible. The nearest terrestrial 
analogy is when a cannon-ball bounds off the sea. In glancing collisions fracture will 
probably not be very frequent. 
From these arguments, it is probable that, when two meteorites meet, they attain 
an effective elasticity of a high order of perfection ; but there is, of course, some loss 
of energy at each collision. [It must, however, be admitted that on collision the 
deflection of path is rarely through a very large angle. But a succession of glancing 
collisions would be capable of reversing the path ; and, thus, the kinetic theory of 
meteorites may be taken as not differing materially from that of gases. 
Perhaps the most serious difficulty in the whole theory arises from the fractures 
which must often occur. If they happen with great frequency, it would seem as if 
the whole swarm of meteorites would degrade into dust. We know, however, that 
meteorites of considerable size fall upon the Earth; and, unless Mr. Lockyer has 
misinterpreted the spectroscopic evidence, the nebulrn do now consist of meteorites. 
Hence, it would seem as if fracture was not of very frequent occurrence. It is easy 
to see that, if two bodies meet with a given velocity, the chance of fracture is much 
greater if they are large, and it is possible that the process of breaking up will go on 
only until a certain size, dependent on the velocity of agitation, is reached, and will 
then become comparatively unimportant. 
When the volatilised gases cool, they will condense into a metallic rain, and this 
may fuse with old meteorites whose surfaces are molten. A meteorite in that 
condition will certainly also pick up dust. Thus, there are processes in action 
tending to counteract subdivision by fracture and volatilisation. The mean size of 
meteorites probably depends on the balance between these opposite tendencies. If 
this is so, there will be some fractures, and some fusions, but the mean mass will 
change very slowly with the mean kinetic energy of agitation. This view is, at any 
rate, adojrted in the paper as a working hypothesis. It was not, howmver, possible 
to take account of fracture and fusion in the mathematical investigation, but the 
meteorites are treated as being of invariable mass. 
§ 2. On the Velocity of Agitation of Meteoi ites, and on its Secular Change. 
The velocity with which the meteorites move is derived from their fall from a great 
distance towards a centre of aggregation. In other words, the potential energy of 
their mutual attraction when widely dispersed becomes converted, at least partially. 
* Added Nov. 16, 1888. 
