Casagrande liquid-limit apparatus, 1967, and cone penetrometer, 1980s.

In 1932, Arthur Casagrande described a device (left) to measure the liquid limit of a soil sample (the point at which the soil begins to flow). This is done by observing how quickly a groove marked in a soil sample closes up after the cup containing it has been repeatedly jolted. The one shown here is hand-operated and has a revolution counter to 'BS 1377' and was made by Leonard Farnell & Co Ltd. Used since the 1970s, the cone penetrometer (right) measures the resistance of the soil to a metal cone. The liquid limit can be plotted by measuring the resistance offered by soil samples with different water contents.