Discuss about the Hydrometer Method to determine soil

Bouyoucos devised a simple and rapid technique using a specially designed hydrometer to determine sand, silt and clay content of soil without separating them. In this method a known amount of the air dry soil (say 50 g of fine soil) is dispersed in a known amount of water containing calgon (sodium hexametaphosphate) solution. After a certain period of time, in this case 40 seconds, when the largest particle (the sand) will have settled on the bottom of the cylinder, the density of the liquid is determined with the help of a hydrometer.  This gives a measure of the silt and clay in the soil. So, the 40 sec reading is the reading "grams of silt and clay in suspension".  Similarly, a density measurement is performed at two hours when the silt would also have settled. This two hour reading reads "grams of clay". The amount of sand is determined by subtracting the 40 second reading from the total amount.