# Man's physical universe

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216 PHYSICAL LIMITATIONS HA\L BKKX OVKRCOME

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The hydraulic press is used for many purposes in industry: it is used

to compress cotton or paper into bales, to extract the oil

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Fig. 69.

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The hydraulic press

principle.

from cotton

seed, and in machines to test the

strength of various materials.

Hydraulic automobile brakes and

lifts are similar in

hydraulic press.

principle to the

Archimedes' Principle of Floating

Bodies.

Pascal's observations concerning

liquids were the most important

ones made since Archimedes of Syracuse (287-212 B.C.) discovered the

principle, known by his name, that when a body is immersed in a liquid

{or a gas) the loss in weight of the body will be equal to the weight of the

liquid {or gas) displaced, and the body will be buoyed up by a force equal

to the weight of the liquid {or gas) displaced by it.

The story of this discovery is that Archimedes was given the problem

of determining whether or not King Hiero's golden crown had been

alloyed with silver. Archimedes was bathing one day and noticed that

he displaced water equal in volume to his own body. At once he saw

that the lighter alloy of gold and silver would displace more water

than an equal weight of pure gold.

Why do bodies float when immersed in a liquid? What causes a

diver to reach the surface of the water again? The pressure exerted

by a liquid acts on all parts of the surface of a submerged body and

always at right angles to it. Inasmuch as the pressure increases with

depth, there is a greater pressure at the lower surface of the body than

at the upper surface. The difference between these pressures causes

the liquid to exert a buoyant force. The water in the Great Salt Lake

has a greater buoyant effect than that of fresh water because its greater

density produces a greater pressure at any given depth.

Liquids show a wide range of density [mass (weight)

per unit volume].

table

The densities of a few common liquids are given in the following

Substance

Grams per Cubic Centimeter

Gasoline 0.66-0.69

Ethyl alcohol 0.79

Olive oil 0.98

Water 1.00

Chloroform 1.48

Carbon tetrachloride 1.59

Bromine 2.93

Mercury 13.55

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