Examples of historical scientific ideas (no longer considered):
"One idea on offer in the seventeenth century was that comets were a matter of levity: the tail was said to be a stream of levitating material subject to a supposed force of anti-gravity that repelled it from the Sun. Or the comet allegedly stirred up the aether in cosmic space like a ship ploughing through the sea."
Nigel Calder (1980) The Comet is Coming! The feverish legacy of Mr Halley. British Broadcasting Corporation
These days the term 'levity' is used metaphorically, but once it was meant more literally as a kind of opposite of weight.
The reference to a ship is an analogy that is based on the alternative (historical) conception that space is filled with aether.
Read examples of scientific analogies
