Take a drinking straw about 20 cm long. Seal one end with wax, and introduce some lead shot or fine sand until it floats in a vertical position. Then drop in melted wax to keep the shots or sand in place. Have a thin rubber band or a piece of black cotton tied round the stem so that it can be slid up and down as a marker. Put a mark on the straw at the water level. Then take the straw out of the water, and measure the length of the straw from the bottom to the water level mark. Let it be x cm. Now let us assume that water has a specific gravity of unity and that the straw has a uniform area of cross section. Thus we may put a set of markings on the straw for measuring specific gravities of different liquids with ranges, say, from 0.6 to 1.2 by using the formula:

length of straw from the bottom to the mark = (x) / specific gravity of liquid