Twilight. That portion of the morning or evening, in which the light of the sun is perceptible, although he is below the horizon.
Vacuum. Space void of matter. Such is supposed to be the space in which the planets revolve. We are said to produce a vacuum, when we exhaust the air from a receiver.
Valve. A part of a pump, and of some other instruments, which opens to admit the passage of a fluid in one direction, but closes when pressed in the opposite direction, so as to prevent the return of the fluid; a pair of bellows is furnished with a valve.
Vapour. Exhalations from fluid or solid substances, generally mixing with the atmosphere. The most abundant, is that from water.
Vertical. Exactly over our heads: ninety degrees above our horizon.
Vibration. The alternate motion of a body, forwards and backwards; swinging, as a pendulum.
Visual. Belonging to vision; as the visual angle, or that angle formed by the rays of light which enter the eye, from the extremities of any object.
Undulation. A vibratory, or wave-like motion communicated to fluids. Sound, is said to be propagated by the undulatory, or vibratory motion of the air.
Wedge. One of the mechanical powers; the form of the wedge is well known. It is of extensive use; serving to rend bodies of great strength, and to raise enormous weights.
Wheel and Axle. One of the mechanical powers, used under various modifications. Cranes for raising weights, the wheels and pinions of clocks and watches, windlasses, &c. are all applications of this power.