Fahrenheit hydrometer

It was invented by Daniel Gabriel Fahrenheit (1686–1736), better known for his work in thermometry.

In the figure shown here, the hydrometer is floating vertically in a cylinder containing a liquid.

Next, the device is placed in water, and a weight (w) sufficient to sink a marked point on the rod to the water-line is placed on the pan.

At that point, the weight of water displaced by the instrument equals W + w. The hydrometer is then removed, wiped dry, and placed in the liquid whose density is to be determined.

A weight (x) sufficient to sink the hydrometer to the same marked point is placed in the pan.