When I first came across nomograms in my early teens I was mystified by them. They looked interesting but the article on stellar evolution they accompanied gave no clue on how to use them. They looked a bit like graphs, but had three or more graduated scales, and these scales didn't have to be at right-angles to each other, indeed, they might even be curved! It was only a few weeks later that I discovered that you were meant to lay a ruler or straight-edge across the graduated scales, and by doing so you could perform various calculations. Nomograms are a sort of graphical generalization of a slide-rule, and like the slide-rule they became extinct following the introduction of pocket calculators.
Anyhow, these long dormant memories were triggered when I came across the following awesome series of posts by Ron Doerfler at Dead Reckonings: