The Beaufort Scale is used by sailors as a guide to estimate wind speed without the use of instruments. They do this by looking at the effects the wind has upon the waves.
For example, 0 denotes calm seas and 12 cyclone strength winds.
Link
For an easy read about the history of the Beaufort Scale, this link is worth a look.
Image:
Image from website, http://www.feenixx.com/earth-science/Beaufort-Wind-Force-Poster.php
