6. a) The hydrometer is used to detect density.
    b) The purpose of having lead shot at the bottom of the hydrometer is to keep the hydrometer upright.
    c) The hydrometer is calibrated in such a way that the readings of the scale is increasing downwards because 
        it consist of  a tube with a bulb at one end. Lead shots are placed in the bulb to weigh it down and enable 
        the hydrometer to float vertically.