rding to the
circumstances under which it acts: The force which keeps the particles
of matter together to form bodies or masses, is called attraction of
_cohesion_; that which makes bodies stick together only on their
surfaces, is called _adhesion_; that which inclines different masses
toward each other, as the earth and the heavenly bodies, is called
_gravitation_; that which forces the particles of substances of
different kinds to unite, is known under the name of _chemical
attraction_; that which causes the needle of the compass to point
constantly toward the poles of the earth, is _magnetic attraction_;
that which is excited by friction in certain substances, is known as
_electrical attraction_.

How do you know that attraction exists through the whole universe?

This great universal law was first discovered by Sir Isaac Newton. The
sun and planets and other heavenly bodies are only guided in their
path by gravitation.

Do we experience this attraction upon our earth?

Yes; because our earth is carried around the sun by it; and, further,
the tides show it very clearly.

What are the Tides?

The ebbing and flowing of the sea, which regularly takes place twice
in twenty-four hours. The cause of the tides is the attraction of the
sun, but chiefly of the moon, acting on the waters of the ocean.

What is Gravity?

Gravity is the attraction between the earth and the bodies on the
earth, which makes what we call weight of bodies.

What do you understand by specific weight or gravity?

It means the weight of a body as compared with the weight of an equal
bulk of some other body taken as a standard--commonly water.

Why do we say that certain metals--as, for example, platina or
gold--are heavier than others, say, lead or iron?

Because the former have a greater specific gravity.

But is not a pound of gold as heavy as a pound of lead?

Yes; but a lump of gold will be heavier than a lump of lead of equal
bulk.

Can we explain by this what we call floating?

A body will float in water if its gravity is less than that of water;
for example, wood floats for this reason in water, and a balloon in
the air.

Why does a portion of the floating body sink below the surface of the
water?

Because the body in order to float must displace a portion of water
equal in weight to the whole floating body.

But why do iron steamers float--iron being heavier than water?

Because the steamer is not