It is a remarkable fact, that the flow of sand in the hour-glass is
perfectly equable, whatever may be the quantity in the glass; that is,
the sand runs no faster when the upper half of the glass is quite full
than when it is nearly empty. It would, however, be natural enough
to conclude that, when full of sand, it would be more swiftly urged
through the aperture, than when the glass was only a quarter full, and
near the close of the hour.
The fact of the even flow of sand may be proved by a very simple
experiment. Provide some silver sand, dry it over or before the fire,
and pass it through a tolerably fine sieve. Then take a tube, of any
length or diameter, closed at one end, in which make a small hole, say
the eighth of an inch; stop this with a peg, and fill up the tube with
the sifted sand. Hold the tube steadily, or fix it to a wall, or frame,
at any height from a table; remove the peg, and permit the sand to flow
in any measure for any given time, and note the quantity. Then, let the
tube be emptied, and only half or a quarter filled with sand, measure
again, for a like time, and the same quantity of sand will flow: even
if you press the sand in the tube with a ruler or stick, the flow of
the sand through the hole will not be increased.
The above is explained by the fact, that when the sand is poured into
the tube, it fills it with a succession of conical heaps, and that all
the weight which the bottom of the tube sustains, is only that of the
heap which _first_ falls upon it; as the succeeding heaps do not press
downward, but only against the sides or walls of the tube.