pounds, and if the water
is 10 feet deep, the load borne by each square foot will be 625
pounds. The deeper the water, the greater will be the weight sustained
by the confining vessel and the greater the pressure exerted by the
water.

[Illustration: FIG. 149.--Water 1 foot deep exerts a pressure of 62.5
pounds a square foot.]

Since the pressure borne by 1 square foot of surface is 62.5 pounds,
the pressure supported by 1 square inch of surface is 1/144 of 62.5
pounds, or .43 pound, nearly 1/2 pound. Suppose a vessel held water to
the depth of 10 feet, then upon every square inch of the bottom of
that vessel there would be a pressure of 4.34 pounds. If a one-inch
tap were inserted in the bottom of the vessel so that the water flowed
out, it would gush forth with a force of 4.34 pounds. If the water
were 20 feet deep, the force of the outflowing water would be twice as
strong, because the pressure would be doubled. But the flow would not
remain constant, because as the water leaves the outlet, less and less
of it remains in the vessel, and hence the pressure gradually sinks
and the flow drops correspondingly.

In seasons of prolonged drought, the streams which feed a city
reservoir are apt to contain less than the usual amount of water,
hence the level of the water supply sinks, the pressure at the outlet
falls, and the force of the outflowing water is lessened (Fig. 150).

[Illustration: FIG. 150.--The pressure at an outlet decreases as the
level of the water supply sinks.]

196. Why the Water Supply is not uniform in All Parts of the City.
In the preceding Section, we saw that the flow from a faucet depends
upon the height of the reserve water above the tap. Houses on a level
with the main supply pipes (Figs. 148 and 151) have a strong flow
because the water is under the pressure of a column _A_; houses
situated on elevation _B_ have less flow, because the water is under
the pressure of a shorter column _B_; and houses at a considerable
elevation _C_ have a less rapid flow corresponding to the diminished
depth _(C)_.

Not only does the flow vary with the elevation of the house, but it
varies with the location of the faucet within the house. Unless the
reservoir is very high, or the pumps very powerful, the flow on the
upper floors is noticeably less than that in the cellar, and in the
upper stories of some high building the flow is scarcely more than a
feeble trickle.

[Illustration: FIG. 151.--Water pressur