Scheme of an Areometer
<a href="//commons.wikimedia.org/wiki/User:DnetSvg" title="User:DnetSvg">dnet</a>
self-made, based on <a class="external free" href="https://commons.wikimedia.org/wiki/Image:Aerometer.PNG">http://commons.wikimedia.org/wiki/Image:Aerometer.PNG</a>