Demonstrations. -Tie a strong thread to a stone, suspend the stone from a spring scale, and note its weight. Weigh again, letting the stone hang in a beaker of water, and the scale will be found to read less. Why?

Suspend from one side of a balance a short brass tube A (Fig. 1), and from a hook in the closed bottom of this tube suspend a solid cylinder B, which will just fill the tube. Put weights upon the other scale pan until the beam is horizontal. Immerse B in water, and the equilibrium will be destroyed. Fill A with water, and the equilibrium will be restore We learn from the above, both that a body appears to lose weight when it is immersed in a liquid, and that the amount of this apparent loss is exactly the weight of the water displaced. The fact that a submerged body seems to weigh less in a liquid than in the air was observed by the Greek philosopher Archimedes in the third century в.с. Не not only observed the apparent loss of weight, but discovered the exact law governing it, hence the law is called the Principle of Archimedes. It may be stated as follows:

A body immersed in a liquid is buoyed up by a force equal to the weight of the displaced liquid.

This tendency of a liquid to lift a submerged body is called its buoyancy, and depends in amount upon the density of the liquid and the size of the body.

liquid and the size of the body. Since weight is the measure of the mutual attraction between the earth and the body weighed, there can be no real loss of weight, when a body is submerged in water. If, however, we suspend the body by means of a spring scale and weigh it in the air and then weigh it again in water, there will be a decrease indicated on the scale, and it is this decrease that is often called loss of weight.

If a body, a cube for instance, is immersed in a liquid, the horizontal pressure acting upon any side will be exactly counterbalanced by the pressure upon the opposite side. The downward pressure upon the upper surface A will be equal to the weight of a column of water having for its base the area of A, and for its height the depth of A below the surface of the water. The upward pressure upon the lower surface B will be equal to the weight of a column of water having for its base the area of B, and for its height the depth of B below the surface of the water. The difference between these pressures is the buoyancy of the liquid, and is equal to the weight of a quantity of the liquid that has the same volume as the submerged cube. This conclusion is verified by the result of experiment.

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