The Wndsn Telemeter -- OUR Telemeter -- is a distance measuring instrument based on comparing the triangle between our eye and a scale with the triangle between our eye and the object in question.
But there is ANOTHER Telemeter, a widely known and widely distributed (in certain circles that is) Telemeter. It's a special scale that is part of many chronograph watches -- a set of markings that, in its metric version, juxtaposes a scale divided in roughly 20 parts to the minute scale divided in 60 parts. Just like OUR Telemeter, the chronograph Telemeter is used to measure distance (hence the name), but based on a completely different principle, which is, the speed of sound.
The chronograph Telemeter works by comparing the sound to the sight of an event to determine its distance to the observer. We see an event before we hear it and we can use this to time the difference between sight and sound reaching us.
The speed of light is 299,792,458 m/s and for distances on earth, we pretty much see an event instantly, as it happens.
Now, the speed of sound is 343 m/s or 1,235 km/h. This means that in 3 seconds, the sound travels about 1,000 m or 1 km or about 20 km in 60 seconds.
To be accurate, the correct value is 20.58 km in 60 seconds. For miles, we (some watches) have a Telemeter scale of 12.790 mi and for nautical miles, we (some other watches) have 11.112 nm in 60 seconds.
This is it. Take the time from when you see an event to when its sound reaches you and read the value corresponding to the seconds on the Telemeter scale in the unit that Telemeter is calibrated.
The picture features an Omega Speedmaster Chronograph with Tachymeter, Telemeter, and Pulsometer, next to a Wndsn Telemeter on top of a plot of the accurate conversion scales for an acoustic Telemeter in kilometers, miles, and nautical miles.