By ALBERT B. PORTER.
The young student of physics occasionally has difficulty in grasping
the laws of pressure in fluids. His every day experience has taught
him that a push against a solid body causes it to push in the same
direction, and he often receives with some doubt the statement that
pressure applied to a fluid is transmitted equally in every direction.
The experiments ordinarily shown in illustration of this principle
prove that pressure is transmitted in all directions, but do not prove
the equality of transmission, and in spite of all the text books may
tell him, the student is apt to cling to the idea that a downward
pressure applied to a liquid is more apt to burst the bottom than the
side of the containing vessel.
[Illustration: Figs. 1. and 2.]
The little piece of apparatus shown in Fig. 1 was designed to furnish
a clear demonstration of the principle under consideration. It is
essentially an arrangement by which a downward pressure is applied to
a confined mass of air or water, and the resultant pressures measured
in the three directions, down, up, and sideways. By means of a broken
rat tail file kept wet with turpentine three holes are bored through a
bottle, one through the bottom, one through the side, and one through
the shoulder, as near the neck as may be convenient. The operation is
quick and easy, the only precaution to be observed being to work very
slowly and use but a slight pressure when the glass is nearly
perforated. The holes may be enlarged to any size required by careful
filing with the wet file. From each of the holes a rubber tube leads
to one of the glass manometer tubes at the right in the figure, the
joints being made air tight by slipping into each rubber tube a piece
of glass tubing about half an inch long in order to swell it to the
size of the hole it is to fit. The ends of these glass tubes must be
well rounded by partial fusion in a gas flame, that there may be no
sharp edges to cut the rubber. The bottle rests in a depression in the
turned wood base, the lower rubber tube passing out through a hole in
the wood. Fig. 2 shows the shape of the manometer tubes. They are made
of quarter inch glass tubing bent to shape in a flame and left open at
both ends. They are mounted on a scale board which has several
equidistant horizontal lines running across it. The two bent wires
which support the scale board fit loosely in holes in it and in the
base. This method of mounting is very handy, since it permits the
scale board to be swung to right or left as may be convenient, or
turned round so as to show the fittings on its back, without moving
the bottle. The three manometers are filled to the same level with
mercury, the quantity being adjusted by means of a pipette. A
perforated rubber stopper, fitted with a glass tube on which is
slipped a rubber syringe bulb, completes the apparatus.
When the bulb is pinched between the fingers, the mercury is forced up
to the same height in each of the manometers, thus proving that the
pressure is exerted equally in the three directions, up, down, and
sideways. With the bottle filled with water the same effect follows,
the law being the same for liquids and gases. When using water in the
apparatus it is essential that the rubber tubes, as well as the
bottle, be filled, and when used in the class room it is better to
show the experiment with water first, it being easier and quicker to
empty the bottle and tubes than to fill them.
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