From HG Wells’ ‘The Shape Of Things To come’ :

[The air-dollar] was not a metallic coin at all; it was a series of paper notes, which represented distance, weight, bulk, and speed. Each note was good for so many kilograms in so much space, for so many kilometres at such a pace. The value of an air-dollar had settled down roughly to a cubic metre weighing ten kilograms and travelling two hundred kilometres at a hundred kilometres an hour.’