containing the velocity ends of the molecules, let us suppose any origin
for velocities in a unit volume and from this as a pole lay off as vectors the molecular velocities lying between the limits,
where
are the components of said velocities parallel to the respective co-ordinate axes. Then, under the velocity limitations imposed, the end of the velocity of each such molecule will lie in the elementary parallelopiped
, one vertex of this parallelopiped having of course the co-ordinates