The very nature of the sound of running water pronounces its origin
to be the bursting of bubbles: the impact of water against water is a
comparatively subordinate cause, and could never of itself occasion the
murmur of a brook; whereas, in streams which Dr. Tyndall has examined,
he, in all cases where a ripple was heard, discovered bubbles caused by
the broken column of water. Now, were Niagara continuous, and without
lateral vibration, it would be as silent as a cataract of ice. In all
probability, it has its “contracted sections,” after passing which
it is broken into detached masses, which, plunging successively upon
the air-bladders formed by their precursors, suddenly liberate their
contents, and thus create _the thunder of the waterfall_.