deep, and is fed by water coming from the hills of Champagne, which are much higher than Paris. The supply of water from this well is immense.

Many Artesian wells have been sunk in our own country.

IV. PRESSURE ON SUBMERGED BODIES.

Principle of Archimedes.

86. Ir a body is submerged in a fluid, it will be pressed

in all directions, but not equally.

To illustrate, suppose a cube
immersed in water, as -shown in
Fig. 58.
The lateral faces, a and

b, will be equally pressed and in
opposite directions. The same will
be true for the other lateral faces.
Hence, the horizontal pressures will
exactly neutralize each other. The
upper and lower faces, c and d, will
be unequally pressed, and in oppo-
site directions. The face, c, will

Fig. 58.

(86) Are submerged bodies pressed equally in all directions? Illustrate in detail

be pressed upwards by a force equal to the weight of a column of the liquid whose cross-section is that of the cube, and whose height is the distance of c from the surface of the fluid. The face, d, will be pressed downwards by the weight of a column of the liquid,

Cylinder and Bucket Experiment.

88. The principle of ARCHIMEDES may be illustrated by what is called the Cylinder and Bucket Experiment, as shown in Fig. 59. A hollow cylinder or bucket, b, of brass, is attached to the hook of one of the scale pans, and from it is suspended a solid cylinder of brass, just large enough to fill the bucket, and the two are balanced by weights placed in the opposite scale pan. A glass vessel

Enunciate the principle of ARCHIMEDES. (87.) What is a Hydrostatic Balance? Explain its construction. (88.) Explain the Cylinder and Bucket Experiment.

having been placed beneath the cylinder, water is gradually poured into it, until the cylinder is immersed. The opposite scale pan will descend, showing that the cylinder is

buoyed up by some force. If we now fill the bucket, o, with water, the equilibrium will be restored, and the beam will come to a level. Because the water poured into the bucket is equal to that displaced by the cylinder, we infer that the buoyant effort is exactly equal to the weight of the displaced fluid.

The principle of ARCHIMEDES is so called, because it was first discovered by the illustrious philosopher of that name. He was led to the discovery in an attempt to detect a fraud, perpetrated upon

Why is the principle of ARCHIMEDES so called?

HIERO of Syracuse, by a goldsmith, whom he had employed to make a golden crown. The artisan mixed a portion of silver with the gold that was given him for making the crown, but by means of the principle above explained, ARCHIMEDES was able to determine the exact amount of each material employed.

Floating Bodies.-Principles of Flotation.

89. When a body is plunged into a liquid, it is urged downward by its proper weight, and upward by the buoyant effort of the liquid, and, according to the relative intensities of these two forces, three cases may arise:

1. If the density of the immersed body is the same as that of the liquid, its weight will be equal to the buoyant effort of the liquid, and it will remain in equilibrium wherever it may be placed. This is pràctically the case with fishes. They maintain themselves in any position in which they may happen to be, without effort.

2. If the density of the body is greater than that of the liquid, its weight will be greater than the buoyant effort, and the body will sink to the bottom. This is what happens when a stone or piece of iron is thrown into water.

3. If the density of the body is less than that of the liquid, its weight will be less than the buoyant effort, and the body will rise to the surface. The body will continue to rise until the weight of the displaced liquid equals that of the body, when it will come to rest. It is then said to float. Thus, a piece of wood floats upon water, and in like manner a piece of iron floats upon mercury.

When a floating body comes to rest on a liquid, the plane of the upper surface of the liquid is called the Plane of Flotation.

Explain the method of its discovery. (89.) When a body is plunged into a liquid, what three cases may arise? Explain the first case. The second case. The third case. What is the Plane of Flotation?

It sometimes happens that a body which is more dense than a liquid floats upon it. Thus, a porcelain saucer floats upon water. This arises from its form being such, that it displaces its own weight of water, when only partially immersed. For the same reason iron ships float freely on the ocean.

Illustration of the Principles of Flotation.

90. The principles of flotation may be illustrated by an instrument shown in Fig. 60, which under various forms is sold in the shops as a child's toy.

In the form shown, it consists of a high and narrow glass vessel, surmounted by a brass cylinder, A, in which is an air-tight piston that may be raised or depressed by the hand. The vessel is partially filled with water, and contains a light body, as a fish, hollow and of porcelain or glass. The fish is attached to a sphere of glass, m, filled with air, and with a small hole, o, at its lower side, through which water can flow in or out, as the pressure is increased or diminished.

Under ordinary circumstances the sphere, m, with its attached fish,

floats at the surface of the water. If the piston is depressed, the air beneath it is compressed, and acting upon the water forces a portion of it into the globe. The apparatus then becomes more dense than the water, and sinks. By relieving the

Fig. 60

pressure, the air in the globe expands and drives the water out, when it again floats on the surface. The operation may be repeated at pleasure.

Explain the case of a dense body floating on a liquid. (90) What instrument illustrates the laws of flotation? Explain its use and action.