GAS. Any kind of air; of these there are several. The atmosphere consists of two kinds, mixed, or combined with each other. GEOMETRY. That branch of the mathematics, which treats of lines, of surfaces, and of solids; and investigates their properties, and proportions. GLOBE. A sphere, or ball. It has a point in its centre of magnitude, from which its surface is every where equally distant. GRAVITY. That species of attraction which appears to be common to matter, existing in its particles, and giving to them, and of course to the masses which they compose, a tendency to approach each other. By gravity a stone falls to the earth, and by it the heavenly bodies tend towards each other. HARMONY. A combination of musical sounds, produced by vibrations which bear a certain ratio to each other; and which thence affect the mind agreeably, when heard at the same time. Sounds not so related, produce discord. HEMISPHERE. Half a sphere or globe. A plane passing through the centre of a globe, will divide it into hemispheres. HORIZON. This is generally divided into _sensible_, and _rational_. The sensible horizon is that portion of the surface of the earth, to which our vision extends. Our rational horizon is that circle in the heavens which bounds our vision, when on the ocean, an extended plane, or any elevated situation. In the heavens our sensible, and our rational horizon are the same; its plane would divide the earth into hemispheres at 90 degrees from us; and a person standing on that part of the earth which is directly opposite to us, would, at the same moment, see in his horizon, the same heavenly bodies, which would be seen in ours. HORIZONTAL. Level; not inclined, or sloping. A perfectly round ball, placed upon a flat surface, which is placed horizontally, will remain at rest. HYDRAULICS. That science which treats of water in motion, and the means of raising, conducting, and using it for moving machinery, or other purposes. HYDROSTATICS. Treats of the weight, pressure, and equilibrium of fluids, when in a state of rest. HYDROMETER. An instrument used to ascertain the specific gravity of different fluids, which it does, by the depth to which it sinks when floating on them. IMAGE. The picture of any object which we perceive either by reflected or refracted light. All objects which are visible, become so by forming images on the retina. IMPENETRABILITY. That property of matter, by which it excludes all other matter from occupying the same space with itself at the same time. If two particles could exist in the same space, so also might any greater number, and indeed all the matter in the universe, might be collected in a single point. INCIDENCE. The direction in which a body, or a ray of light, moves in its approach towards any substance, upon which it strikes. INCLINED PLANE. One of the six mechanical powers. Any plane surface inclined to the horizon, may be so denominated.