CONDENSERS. BY F. R. If you build up a solid column of bricks the
pressure which it exerts on its base will increase directly as the
height of the column. A column ten feet in height will press twice
as hard on its base as a column five feet high, and a column 100
feet high ten times as hard as a lo-foot column. Now, the point I
want to make is that the pressure per square inch of base depends
altogether on the height and not on the width or diameter of the
column. A column 2 feet square will, it is true, press on its base
with four times the pressure of a column one foot square and of the
same height, because there are four times as many bricks in it and
it weighs four times as much, but there is also four times as much
base to it, so that the pressure per square inch of base is
entirely independent of the cross section and depends upon the
height alone. The same thing is true of water. A cubic foot of
fresh water weighs 62.355 pounds at 62 degrees Fahrenheit. It is
easy to remember this weight approximately, for it is the same as
the de- with water. A grees and 62 is a standard temperature in
dealing cubic foot restson a base of 144 square inches and is a
foot high, so that the pressure per square inch on the base would
be 62-355 -- 144 -433 of a pound and for every foot in height that
we build our column or fill our pipe with water we gain 0.433 f a
pound pressure per square inch. If one foot or 12 inches gives us
0.433 of apound it would take a column 12 -f- .433 27.71 inches in
height to exert a pressure of one pound per square inch. For 82964
Fluid Pressure is Dependent upon Height of Column. every 27.71
inches in vertical height between the point at which you are
measuringand thetop of a column of still water there will be a
pressure of a pound to the square inch, and it makes no dif-
ference whether you are measuring the pressure at the bottom of a
one-eighth inch pipe, a twenty foot stand-pipe, or a lake, or the
ocean itself. Every once in a while we have to explain this to the
man who believes it takes more power to feed into the bottom of a
tank than into the top, on account of the weight of water in the
tank. The bottom of the tank holds up all the water except the
column directly over the opening of the deliv- ery pipe, so that
the additional pressure on the pump is due only to the depth of
water in the tank, not to the size of the body, and it is
impossible to feed into the top without increasing the height of
the column fully as much. It makes no difference whether the height
is due to the depth of the water inside the tank or an ad- ditional
length of pipe outside. The difference between the water and the
column of bricks is that while the pressure of the latter can act
only vertically that of the water can act in all di- rections so
that as you Fig. i lower a body into the water the pressure upon
its surface in all directions increases one pound per square inch
for every 27.71 inches of depth of water above it. In Fig. i, for
instance, the pressure due to the column of waterPwill act upward
upon the pistonA and sidewise upon the pistonsBand G as well as
downward upon the piston D. We live at the bottom of an ocean of
air. The winds are its currents, we can heat it, cool it, breathe
and handle it, weigh it, and pump it as we would water. The depth
of this atmospheric ocean cannot be determined as positively as
could one of liquid, for the air iselastic and expands as the
pressure decreases in the upper layers. It is variously estimated
at from 30 to 212 miles...
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