Fig. 7.2.3 The neck of the hourglass is the critical gating factor.One of the reasons (other than gravity) why the hourglass exhibits an even flow of sand is
that the grains of sand are small and uniform in size. Consider what would happen if the
hourglass was to have pebbles inserted in among the grains of sand.
Fig. 7.2.4 shows what happens if pebbles are inserted into the hourglass along with the
grains of sand.
Chapter 7.2: The Standard Work Unit