Page:Mars - Lowell.djvu/78

 As gravity depends upon mass, the larger the attracting planet the greater is its critical velocity, the velocity it can just control; and, reversely, the smaller the planet the less its restraining power. With the Earth the critical velocity is six and nine tenths miles a second. If any of us, therefore, could manage to acquire a speed greater than this, socially or otherwise, we could bid defiance to the whole Earth, and begin to voyage on our own account through space.

This speed is actually attained, as we have seen, by the molecules of hydrogen. If, therefore, a molecule of free hydrogen were present at the surface of the Earth, and met with no other gas attractive enough to tie it down by uniting with it, the rover would, in course of time, attain a speed sufficient to allow it to bid good-by to Earth, and start on interspacial travels of it own. That it should reach its maximum speed is all that is essential to liberty, the direction of its motion being immaterial. To molecule after molecule would come this happy dispatch, till the Earth stood deprived of every atom of free hydrogen.

Now, it is a highly significant fact that there is no free hydrogen found in the Earth’s atmosphere. There is plenty of it in the captivity of chemical combination, but none in the free