devexity
A bending downward; declivity.
uncountable
'Twas the more surprizing upon this account, because since the Matter of thie Phaenomenon seem'd to be but a very Thin Vapour, it was necessary that That Vapour shou'd be either of a vast Extent to answer the Superficies of the Earth; or else of a vast Height, that the Devexity of the Subperficies of the Earth shou'd not hinder its being seen in the same Situation from Places so far distant;
Where it comes from
From Latin devexitas, from devexus. See devex (adjective).
Adapted from the Wiktionary entry devexity (CC BY-SA 4.0).