Equinox. The two periods of time at which the nights and days are every where of equal length. The vernal equinox is in March, when the sun enters the sign Aries; the autumnal equinox in September, when the sun enters Libra. At these periods, the sun is vertical at the equator.
Exhalations. All those articles which arise from the earth, and mixing with the atmosphere, form vapour.
Expansion. The same as [dilatation], which see.
Extension. One of the essential properties of matter; that by which it occupies some space, to the exclusion of all other matter.
Figure. All matter must exist in some form, or shape; hence figure is deemed an essential property of matter.
Fluid. A form of matter, in which its particles readily flow, or slide, over each other. Airs, or gases, are called elastic fluids, because they are readily reduced to a smaller bulk by pressure. Liquids, are denominated non-elastic fluids, because they suffer but little diminution of bulk, by any mechanical force.
Focus. That point in which converging rays unite.
Force. That power which acts upon a body, either tending to create, or to stop motion.
Fountain. A jet, or stream of water, forced upwards by the weight of other water, by the elasticity of air, or some other mechanical pressure.
Friction. The rubbing of bodies together, by which their motion is retarded. Friction may be lessened, but cannot be destroyed.