Learning systems

Le plongement vectoriel is the entry with a metal hidden inside it. Plonger does mean to dive, but the verb was built on Vulgar Latin plumbicare, to sound the depth with a lead, from plumbum, the lead itself — so a plongement is at bottom a weight let down on a line to find out how deep the water is. English owns four members of that family and hears the kinship in none of them. « Plumb » is the lead on the mason’s string, and « to plumb the depths » still does exactly what the sailor did. « A plumber » was the man who worked the lead pipes. « To plunge » crossed the Channel from French in the Middle Ages with the picture already fading. And « a plummet » was the weight itself long before it became the fall. Once that is heard, the mathematical sense follows easily: an embedding lets a word down into a space of numbers and reads off where it comes to rest. The French term keeps the older picture alive in a field where nothing else is old.