Velocities, great, how arrested, 204 Vibration of sounding bodies, 249 Vision, distinct, what necessary to, 354 Vision, why single, 357 Vision, why erect, 356 Visual angle, 348 Voice, how produced, 265 Voltaic electricity, 394 Volta's pile, 395 Walrus, feet of, 157 Water, crystallization of, 63 Water man's first mirror, 112 Waves, how formed, 199 Waves, height of, 200 Wedge, 242 Weight, 52, 82-85 Wheel and Axle, 235 Wheel-barrow a lever, 231 Whispering galleries, 261 Windlass, 236 Windows, double, 303 Winds, 283 Winds as affected by the rotation of the earth, 284 THE END. FOOTNOTES: [1] I take the following from Dr. Arnot: The reasons that so many people are drowned in ordinary cases, who might easily he saved, are the following: 1. Their believing that continued exertion is necessary to keep the body from sinking, and hence their generally assuming the position of a swimmer, in which the face is downward, and the whole head must be kept out of the water to allow of breathing. Now as a man can not retain this position without continued exertion, he is soon exhausted, even if a swimmer, and if not, the unskillful attempt will scarcely secure for him even a few respirations. The body raised for a moment by exertion above the natural level, sinks as far below when the exertion ceases; and the plunge, by appearing the commencement of a permanent sinking, terrifies the unpracticed individual, and renders him an easier victim to his fate. 2. From a fear that water entering by the ears may drown as if it entered by the nose or mouth, a wasteful exertion is made to prevent it; the truth being, however, that it can only fill the outer ear, or as far as the membrane of the drum, and is therefore of no consequence. 3. Persons unaccustomed to the water, and in danger of being drowned, generally attempt in their struggle to keep their hands above the surface, from feeling as if their hands were tied while held below; but this act is most hurtful, because any part of the body kept out of the water, in addition to the face, which must be so, requires an effort to support it which the individual is supposed at the time incompetent to afford. 4. Not having reflected that when a log of wood or a human body is floating upright, with only a small portion above the surface, in rough water, as at sea, every wave in passing must cover the head for a little time, but will again leave it projecting in the interval. The practiced swimmer chooses this interval for breathing. 5. Not knowing the importance of keeping the chest as full of air as possible; the doing which has nearly the same effect as tying a bladder of air to the neck, and without other effort will cause nearly the whole head to remain above the water. If the chest be once emptied, and if from the face being under water the person can not inhale again, the body is then specifically heavier than water, and will sink.