A tachymeter is a scale which measure speed. It does this by is located on an a analog watch. They can be purchased from any store which sells watch supplies

http://www.answers.com/tachymetre

The tachymeter will tell you how many times something will happen in one hour. For instance, if you time a car going around a one mile track, the tachymeter will tell you that the car was traveling at X mph. If the track is only a half mile, then divide by 2 to get mph (and etc. for other distances).

You can use it to tell how many times any event happens in an hour. Time how long it takes for a machine to produce 1 widget and the tachymeter will tell you how many it makes per hour.

It can also be used to compute distance if you know your rate of speed. Say you're on a plane that you know is travelling at a constant 400 mph. Start timing at one point and stop when you want to know the distance. If your tachymeter stops at 100, you know you've traveled four miles. 400 mph/100 = 4.

Hans Knoop has written:

'Leistung und Anwendung von automatisch registrierenden elektronischen Tachymetern' -- subject(s): Data processing, Surveying, Tachymeter

Using a graduated ruler, a surveyors wheel or trundle wheel, or by using an optical (Tachymeter) or laser range finder are methods that may be used to find length or distance.