Man's physical universe

xanabras

158 THE EARTH AS MAN'S ABODE

type of instrument employs a small mirror mounted on a balanced

weight, which causes a beam of light to move on a moving piece of

photographic paper. The problem of the high cost of developing the

photographic paper has recently been eliminated by the use of a photoelectric

cell,' which sets up a fluctuating electric current as the beam

of light flickers across it. This current, amplified by a vacuum-tube

hookup, drives a pen which records the wave on a plain white paper.

The essential part of the seismograph is like a pendulum, the bob of

which tends to remain stationary in space, while everything attached

to it moves. It is impossible to attach the pendulum in such a way that

it docs not move to a certain extent with the earth, but this motion may

be corrected so as to obtain the true motion of the earth relative to it.

It has been found that the vibrations produced by earthquakes pass

through the center of the earth. The rate at w^hich vibrations pass

through different media is quite w^ell know^n; and from this information

it is concluded that, if the center of the earth is a molten liquid, these

vibrations would travel much less rapidly than they actually do travel.

Data obtained with the seismograph and other lines of evidence suggest

that the earth has an outer layer of soil and rock about 37 miles

thick. Beneath this layer there is thought to be a layer of compounds

of iron, magnesium, and silicon about 950 miles thick. Below this

there is another similar layer about 875 miles thick, in w^hich the iron

content increases with the depth. The center of the earth is thought to

consist of a core of iron and nickel, which accounts for its magnetism.

The other elements are found throughout these layers in relatively

insignificant quantities. If these ideas are correct, it seems quite

possible that the earth may have been in a molten condition at one

time, these substances being separated in layers in accordance with

their densities.

Earthquakes are recorded by seismographic stations throughout the

world. The deformation of the earth under the influence of stresses

which develop within it frequently takes the form of sudden fractures.

Displacements at the surface are produced by these fractures, which

start vibrations that spread throughout the earth. Inasmuch as any

earthquake is made up of three linear displacements along directions at

right angles to each other, a fully equipped observing station requires

three seismographs, set to read the north-south, east-west, and vertical

components of the earthquake. A rough estimate of the position of the

earthquake origin can be obtained by stations thus equipped, without

the use of records from other stations.

Although the primary causes of earthquakes lie within the earth, it

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This topic will be studied in a later unit.

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