The common method of determining the specific gravity of liquids is by means of the hydrometer (Fig. 52). This instrument consists of a glass tube with mercury or lead shot in the bottom to keep it in the water. A scale is graduated on the narrow stem reading either directly or indirectly into specific gravity reading. The scale is graduated by placing it in liquids of known strength and marking the level of the liquid on the stem. It is usual to have two separate instruments, one. for light liquids, on which the mark is near the bottom of the stem, and one for heavy liquids, on which it is near the top.
Fig. 52. - Hy drometer.