or GRAVITATION (for the words are most commonly used synonymously), signifies either the force by which bodies are pressed towards the surface of the earth, or the manifest effect of that force; in which last sense the word has the same signification with weight or heaviness.

Concerning gravity in the first sense of the word, or that active power by which all bodies are impelled towards the earth, there have been great disputes. Many eminent philosophers, and among the rest Sir Isaac Newton himself, have considered it as the first of all second causes; an incorporeal or spiritual substance, which never can be perceived any other way than by its effects; an universal property of matter, &c. Others have attempted to explain the phenomena of gravitation by the action of a very subtle etherial fluid; and to this explanation Sir Isaac, in the latter part of his life, seems not to have been adverse. He hath even given a conjecture concerning the matter in which this fluid might occasion these phenomena. But for a full account of the discoveries of this great philosopher concerning the laws of gravitation, the conjectures made by him and others concerning its cause, the various objections that have been made to his doctrine, and the state of the dispute at present, see the articles NEWTONIAN Philosophy, ASTRONOMY, ATMOSPHERE, EARTH, ELECTRICITY, FIRE, LIGHT, ATTRACTION, REPULSION, PLenum, VACUUM, &c.

Specific Gravity, denotes the weight belonging to an equal bulk of every different substance. Thus the exact weight of a cubic inch of gold, compared with a cubic inch of water, tin, lead, &c. is called its specific gravity. See HYDRASTICS.