A correspondent wrote: “One of my friends asked me why we say something has a ‘fifty mile’ radius instead of saying it has a ‘fifty miles’ radius. Do you know why we drop the ‘s’ in that situation?” I responded:
The example you give is an adjective formation, and in that context the singular is always used in English, whether it’s a unit of measure or not: a three-country pact, a ten-gallon hat, a six-man crew, etc. I’m not sure what the accepted historical analysis is, but it occurs to me that it might be a generalization from the situation with units of measure, in which people used to use what looked like a singular in all contexts: “It’s five mile to the next town,” “The water level is eight foot down,” etc. Now, in that context it’s a remnant of the Old English genitive plural. For instance, the plural of fōt ‘foot’ was fēt, which became feet, but the genitive plural (‘of feet’) was fōta, which became foot just like the singular. My guess is that in adjectival constructions, the apparent singular was generalized to all nouns, perhaps partly because it’s easier to say without the plural -s, but otherwise the plural was generalized (though there are still people who say “eight foot down” and so on).
Rather than actually do some research to try to find out what the accepted explanation is, I had the bright idea of tossing the question out there for you knowledgeable readers to deal with. Anybody know?