Yema was a French maker of high quality watches. It was founded in 1948. The first Yema slide rule I'm aware of is this 1969 meangraf model 61 007 S (visible in the lower left of the following advertisement). Meangraf apparently means "hourly average meter." It's a rally style watch with a logarithmic scale that rotates around a fixed time scale, but the scale markings are a bit unusual. The outer scale is marked in kilometers and the inner scale is marked in minutes. The minute markings are a bit strange. One hour and twenty minutes is marked 120. 200 means 2 hours exactly. Like any rally style watch, you turn the bezel to line up the distance you've traveled on the outer scale with the time it took you to travel that distance on the inner scale. Your average speed is the number at 9 o'clock, directly across from 1 hour on the time scale.