A telemeter scale enables the calculation of the distance between an acoustic signal and its own position. Or, put more simply, it can be used to determine the proximity of a storm. To do this, the chronograph is started when lightning strikes and stopped at the first clap of thunder. It is then possible to gauge how far away the storm is by reading the telemeter scale and using the second hand as a counter. The scale is based on the well-known sonic speed value (343 m/s or 1,235 km/h), and was originally employed in a military context. It was used to determine the enemy's position via muzzle flashes and cannon fire.