Because they are influenced by the attraction of gravitation, by which all bodies are drawn towards the centre of the earth.
775. Why do bodies lighter than the air ascend?
Because the air, being a denser body, obeys the law of attraction, and in doing so displaces lighter bodies that interfere with its gravitation.
776. Why do fragments of tea, and bubbles floating upon the surface of tea, draw towards each other, and attach themselves to the sides of the cup?
Because they are influenced by the attraction of cohesion.
Cohesion.—The act of sticking together.
777. Why will a drop of water upon the blade of a knife leave a dark spot?
Because the iron of the knife attracts the oxygen of the water, by chemical affinity; and the two substances form a thin coating of oxide of iron.
Affinity.—Attraction between dissimilar particles through which they form new compounds.
778. Why do clouds sometimes move towards each other from opposite directions? and