Original poster: dave pierson <davep-at-quik-dot-com>
I would not propose to set a date on 'knowledge of the ionosphere'.
It was calculated, (and, i think, observed) as early as 180 ('pre Tesla's
Colorado Springs observations) that there was a conductive
layer, based on observations of 'reflections' and
resonances.
The 6.mumbleish Hz now named after Schumann
dates to that date at least..
best
dwp