Here’s a photo of Marconi in 1897. He wasn’t sure how Hertzian waves travel, or the waves he generated with this apparatus either, which he suspected were different from those manufactured by Hertz. Nobody really understood them, not even with the benefit of Maxwell’s equations. There was this ether business, a cypher, that people didn’t necessarily believe in but which explained things about invisible propagation that they could observe; ether was rather like dark matter now, vis-à-vis our galaxy’s not flying apart as it should do.
What was known about the ether, as of 1897? Quite a lot. First, it was understood to have a density expressible on the order of 10 to the minus 27th, calculated somehow ‘from the energy with which the light from the sun strikes the earth.’ It is a substance so fine that atoms sit like marinated cherries within it. It perfuses all matter in the universe, so that all physical things are in continuity with all other physical things. Yet things are not continuous with the ether. Vibrate the ether, and the vibration will pass through distant objects – but the objects themselves will not vibrate. The earth does not push through the ether, like a boat; the ether allows the earth to pass, as water allows a moving sieve to pass. Rays, of light or electricity or Röntgen beams, move as vibrations of the ether. For reasons not yet understood, the ether will conduct every kind of ray through every kind of substance. Nor does it conduct rays at the same speed through every substance. The ether in glass carries light at about 120,000 miles per second; through the air, closer to 192,000. Glass alone, with no ether inside it, allows light to pass at a pokey 3 miles per second.
Who says: H.J.W. Dam, “Telegraphing without Wires. A Possibility of Electrical Science,” McClure’s Magazine, March, 1897, pp. 383-92.