their necks through a densely-clothed part of the wood,
when the sound above referred to increased, attracting
the attention of both parties. In a few seconds the
air was filled with a steady and continuous rumbling
sound, like the noise of a distant cataract. Pursuers
and fugitives drew rein instinctively, and came to a
dead stand; while the rumbling increased to a roar, and
evidently approached them rapidly, though as yet nothing
to cause it could be seen, except that there was a dense,
dark cloud overspreading the sky to the southward.