PDF 
HOW TO USE A HYDROMETER
The hydrometer is placed in a glass tube known as a hydrometer jar and which is filled with the liquid that is to be measured. The specific gravity of the liquid is indicated when the level of the liquid in the jar aligns with a point on the hydrometer scale. If the liquid is heavier than water then the hydrometer sits higher in the liquid and gives a higher reading.
Conversely a liquid lighter than water sits higher in the liquid and gives a lower reading.
