: What is the tachymeter function on the outer
: dials on some heuers. What do they measure
: and how do they measure?

Omega's have them too, here's an explanation from a 1957 Omega Catalog:

But to answer your specific queries:

What do they measure

As simply put as possible: Unit's per hour based on measurement of 1 interval that takes from 6-60 seconds.

and how do they measure?

A scale is created by dividing by 3600 by the number of seconds... So 3600 / 60 = 60, 3600 / 45 = 80, 3600 / 30 = 120 for example, and placed it on the watch. Then one can figure an average hourly rate by timing one increment (be it one mile traveled, one Kilometre, or one envelope stuffed) and reading it off the Tachymetre scale...

Your next question... Why 3600? 60 seconds in a minute, 60 minutes in an hour... 60 x 60 = 3600 ...

Pretty easy if you think about it, pretty clever too!

-- Chuck