; this is then covered with a heavy canvass or tarpaulin, and Rods 1, 2, 3, etc., are run through it in sufficient numbers to hold it in position when the area,
, has been back-filled with dry sand. The tarpaulin is then keyed up tight by the rods bearing on washers of large area on the top plane,
. If the sand in the area,
, be then removed, the observer is asked to note whether, in his judgment, the resulting pressure is greater at
or at