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Hydraulic Tourniquet.

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not merely in the direction of this force, but horizontally, and also upwards, as will now be demonstrated.

78. Lateral pressures. Hydraulic tourniquet.-The existence of lateral pressures which liquids exert upon the sides of the vessel in which they are contained,

may be demonstrated by means of the hydraulic tourniquet or Barker's mill (fig. 59). This consists essentially of a long glass tube, C, with a funnel, D, at the top. The bottom of the tube fits into a hollow brass box, which rests on a pivot; in the sides of the box are fitted four brass tubes, arranged crosswise, and all bent in the same direction at the ends.

B

c

Water descending the long tube emerges by the apertures of the bent tubes, which are soon seen to rotate rapidly in the direction indicated by the arrow. This rotation is due to the lateral pressure exerted by the column of water in the long tube. For let us consider one of the bent tubes, aA, Bb, represented in section on the left (fig. 59), and suppose first that the orifices, a and b, are closed. The column of water which then fills the tube C exerts upon the portions of the opposite sides, A and a, equal and contrary pressures which hold each other in equilibrium; this is also the case at B and b, and thus no rotation can be produced in either direction. But if the orifices a and b are open, as is the case when the apparatus is at work, as the water issues by these orifices, the pressures at a and b no longer exist; while those transmitted to A and B continuing to act, produce the rotation. Rotating fireworks also

DEBRAINE. DEL

Fig. 59.

act on the same principle as Barker's mill; that is, an unbalanced reaction from the heated gases which issue from openings in them gives them motion in the opposite directions.

It is owing to the lateral pressure of liquids that dykes and banks which retain rivers or reservoirs, sometimes give way, being too weak for the pressure they have to support.

79. Vertical upward pressure. The pressure which the upper layers of a liquid exert on the lower layers causes them to exert an equal reaction in an upward direction, a necessary consequence of the principle of transmission of pressure in all directions.

The following experiment (fig. 60) serves to exhibit the upward

pressure of liquids. A large open glass tube, one end of which is ground, is fitted with a ground glass disc, a, or still better, with a thin card or piece of mica, the weight of which may be neglected. To the disc is fitted a string, b, by which it can be held against the bottom of the tube. The whole is then immersed in water, and the disc does not fall, although no longer held by the string; it is consequently kept in its position by the upward pressure of the water. If water be now slowly poured into the tube, the disc will only sink when the height of the water inside the tube is equal to the height outside. It follows thence that the upward pressure on the disc is equal to the pressure of a column of water, the base of which is the internal section of the tube a, and the height the distance from the disc to the outer surface of the liquid. Hence the upward pressure of liquids at any point is governed by the same laws as the downward pressure.

Fig. 60.

This upward pressure is termed the buoyancy of liquids; it is perceived when the hand is plunged into water, and still more distinctly if it is immersed in mercury, which being of greater density produces greater pressure. It is owing to this buoyancy that, if a hole be made in the bottom of a ship, water enters with force.

80. Pressure is independent of the form of the vessel.-The pressure exerted by a liquid, in virtue of its weight, on any portion

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Pressures on Liquids.

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of the liquid, or on the sides of the vessel in which it is contained, depends on the depth and density of the liquid, but is independent of the form of vessel and of the quantity of the liquid.

This principle, which follows from the law of the equality of pressure, may be experimentally demonstrated by many forms of apparatus. The following is the one most frequently used, and is due to Masson. It consists of a large conical vessel, M, screwed to a brass tubulure, c, fixed to a wooden support (fig. 61). This tubulure is closed by a disc, a, which does not adhere to it, but is

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simply applied against the edge, and is kept there by a string attached to one end of an ordinary balance, to the other end of which is a scale-pan. Weights are placed in the latter, so as just to counterbalance the pressure of the water on the disc, when the vessel M is almost full; water is then gradually added until the disc just begins to give way and allows some to escape. A rod, o, is then lowered until its point just grazes the surface of the liquid. If the vessel M be unscrewed and replaced by the cylindrical tube, P, the capacity of which is far less, on gradually pouring water in, the moment the level of the liquid just touches the point of the rod, o, the disc, a, begins to allow some water to

escape. The same result ensues if for the straight tube, P, the inclined one, Q, be substituted. In these three cases, therefore, provided the height of the liquid is the same, the pressure is equal on the disc, a, whatever be the shape and capacity of the vessels.

Moreover, the weight which has to be put on the scale-pan to establish equilibrium, shows that the pressure exerted by the liquid is equal to the weight of a column of water, the base of which is the internal section of the tubulure, c, and the height the vertical distance from the disc to the surface of the liquid.

This principle is sometimes called the hydrostatical paradox, for at first sight it seems quite impossible.

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81. Pascal's experiment. Pascal made the following experiment, which proves what great pressures may be produced by even small quantities of liquid when contained in vessels of great height. He fixed firmly, in a stout cask, as represented in fig. 62, a very narrow tube about 30 feet in height, and then filled the cask and the tube with water., The effect of this was to burst the cask; for there was a pressure on the bottom of the cask equal to the weight of a column of water whose base was the bottom itself, and whose height was equal to that of the water in the tube (80).

82. Hydraulic press. The law of the equality of pressure has received a most important application in the hydraulic press, a machine by which enormous pressures may be produced. Its principle is due to Pascal, but it was first constructed by Bramah in 1796.

Fig. 63 represents an elevation, and fig. 64 a section of the instrument; it consists of two iron cylinders or barrels, A and B, of unequal diameters. In the barrel A, which is of very small diameter, is a cylindrical rod, a, which acts as piston, and can be moved up and down by the lever, O. In the cylinder, B, the internal diameter of which

DUJARDIN

Fig. 62.

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is 12 to 15 times that of the barrel, A, is a long cylindrical iron ram, C, which also forms a piston, and works water-tight in the barrel B. On the top of the ram, C, is an iron slab, K, which

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rises and falls with it.

Four wrought-iron columns support a second plate, MN, which is fixed. The objects to be pressed are placed between K and MN.

When the piston is raised by means of the lever, a vacuum is produced in the barrel A, and a valve, S, at the bottom opens and allows water to pass from a reservoir, P, into the barrel. When a re-descends, the valve, S, closes; but another valve, m, placed at the bottom of the tube d, opens; the water is thus forced by this tube into the large cylinder, B. At the next stroke of the piston, a, a fresh quantity of water is drawn from the reservoir, P, and forced into the barrel B, and so forth.

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