[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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PEAR DUCHESSE D’ANGOULEME.