WATER PBESSUKS

363

Pressure is somewhat different from density. The pressure of the
water is exerted equally in every direction, upwards, downwards and
sideways. The pressure of water increases at the rate of 64 Ibs. per
square foot of area for every foot depth of water. Think of a container,
a square box on a table measuring 1 ft X1 ft X 1 ft. so that the area
of each side will be exactly I square foot. Fill the box with water.

Fig 3.
The water is now pressing outwards on every side of the box. The
mean pressure on each side is 32 Ibs. per square foot, but this pressure is
resisted by the strength of the container. The water presses downwards
on the bottom of the box with a mean pressure of 64 Ibs. per square
foot, but this pressure is supported by the upward force of reaction
exerted by the table. There is only the pressure oi tne atmosphere
acting on the surface of the water.
If another exactly similar box be placed on top of the first the press-
ure exerted by the water on its sides and bottom will be the same as
in the case of the lower box.
But what is now the effect on the lower box? Obviously the super-
imposed weight does not add to the pressure exerted by the water in
the lower box, but the additional 1 cubic foot of water does increase the
downward pressure on, the bottom of it which has now to support 2
cubic feet of water, or 2 X 64=128 Ibs., neglecting the weight of the boxes.
The same reasoning would apply to our box when empty and immersed
in water with its top edge on a level with the surface. There would
be no water pressure on the top of the box. The upward pressure at a
depth of 1 foot would be 64 Ibs. because the area of the bottom *is 1
square foot. The mean pressure on each side would be 32 Ibs., because
the mid-point is 6 inches below the surface and -5 ft. X64 Ibs. =32 Ibs.
II the empty container were further immersed so that its top waa
1 foot below the surface then its bottom would be 2 feet down. The