Concrete Matter from Atomic Origins. 405 
of cooling and shrinkage, and possible cavitations, through 
two hundred kilometres below the surface all round the 
globe. It seems indeed quite certain that when the Earth came 
to be almost wholly solid, it must have had in itself some great 
heterogeneousness of constitution or figure, from which its 
present geographical condition had its origin. This hetero- 
geneousness must 'have had its origin in some heterogeneous- 
ness of the primordial distributions of atoms : and we must 
abandon the uniform distribution which we chose in § 9 
merely as an illustration. But we have much more than 
geography to account for. We have to account for : — (1) 
the diurnal rotation of the Earth ; (2) the Earth's motion 
through space, at about thirty kilometres per second, rela- 
tively to the Sun ; (3) the Sun's motion through space 
towards a point in the constellation Hercules, first indicated 
by Sir William Herschel, and more recently estimated at 
about nineteen kilometres per second, relatively to the average 
of sufficiently well observed stars. These three deviations 
from the spherical and irrotational conditions of § § 2 ... 16 are, 
it seems to me, essentially connected with the explanation of 
merely geographical heterogeneousness demanded in §§ 16, 17. 
§ 19. Any system of bodies large or small, or of atoms, 
given at rest, and left subject only to mutual gravitational 
and collisional forces, fulfils throughout all time two laws : — 
Law (1). The centre of inertia of the whole system remains 
at rest. 
Law (2). The sum of moments of momentum* of the 
motions of all the parts, relatively to any axis through the 
centre of inertia, is zero. 
The corresponding laws for a system set in motion in any 
manner, and left to move under the action of mutual forces 
only, are as follows : — 
Law (l x ). The centre of inertia of the whole system moves 
uniformly in a straight line. 
Law (2'). The sum of moments of momentum of the motions 
of all the parts, relatively to any axis through the centre of 
inertia, parallel to any fixed line in space, is constant. 
* (1) The momentum (a name first given in the seventeenth century 
when mathematicians wrote in Latin, and retained in the nineteenth and 
twentieth centuries) of a moving particle is the product of its mass into 
its velocity. 
(2) The moment (a nineteenth century name) of momentum of a 
particle round any axis is the product of its momentum into the shortest 
distance of its line of motion from that axis, into the sine of the inclina- 
tion of its line of motion to that axis. 
(3) The moment of momentum round any axis of any number of 
moving particles is the name given to the sum of their moments of 
momentum round that axis. 
It makes no difference to this definition if any set or sets of the particles 
are rigidly connected to make a rigid body or rigid bodies. 
