The water barometer
Since the density of mercury is 13.6 g/cm ‘, it follows that if water were used as the liquid in a simple barometer the water column would have to be 76 x 13.6 em = 10.3 m long.
Such a barometer was constructed in the seventeenth century by von Guericke and fixed on the outside wall of his house. The upper level of the column was indicated by a small wooden float inside the tube on the surface of the water. With the aid of this barometer von Guericke made the first recorded scientific weather forecast. Having noted a sudden fall in the height of the water column, he correctly predicted the imminence of a severe storm.