Uniforms, do not fit heads, 369. Unique determination, 181-182. Unison, 43. Unit, electrostatic, 111. See _Force_ and _Work_. United States, 336. Universal Real Character, a, 192. Utility of physical science, 351. Variation, the method of, in science, 230; in biology, 216. Velocity, of light, 48 et seq.; of the descent of bodies, 143 et seq.; meaning of, 204; virtual, 149-155. _Verstandesbegriffe_, 199. Vertical, perception of the, 272, 286 et seq.; symmetry, 389. Vertigo, 285, 290. Vestibule of the ear, 300. Vibration, 22 et seq. Vibration-figures, 91. Vinci, Leonardo da, 278, 283. Violent motions, 225. Virtual velocities, 149-155. Visibility, general conditions of, 312. Vision, symmetry of our apparatus of, 96. See _Eye_. Visual nerves, 96. Visualisation, mental, 250. Volt, the word, 343. Volta, 127, footnote, 134. Voltaire, 260. Voltaire's ingènu, 219. Vowels, composed of simple musical notes, 26. Wagner, Richard, 279. Wald, F., 178, footnote. Wallace, 216. War, and peace, reflexions upon, 309, 335 et seq. Waste of mechanical energy, W. Thomson on, 175. Watches, experiment with, 41; in a mirror, 93. Water, jet of, resolved into drops, 60; free, solid figures of, 8; objects reflected in, 94, 191; possible modes of measurement of, 170. Watt, 266. Wealth, the foundation of, 198. Weapons, modern, 335. Weber, 108, 306. Weight of bodies, varies with their distance from the centre of the earth, 112. Weismann, 216. Wheatstone, his stereoscope, 73; his pseudoscope, 96; also 59. Wheel, history and importance of, 61 et seq. Whewell, on the formation of science, 231. Whole, the, 204, footnote. Why, the question, 199, 223. Will, Schopenhauer on the, 190; man's most familiar source of power, 243; used to explain the world, 186; forces compared to, 254; compared to pressure, 14. Windmill, a rotating, 53. Wire frames and nets, for constructing liquid figures of equilibrium, 4 et seq. Witchcraft, 187. Wollaston, 284, 285. Wonderful, science the natural foe of the, 224. Woods, the relative distance of trees in, 68. Wooer, inquirer compared to a, 45. Words and sounds, 343. Words, compared to type, 191. Work, of liquid forces of attraction, 14; in electricity, 173; measure of, 119 et seq., 130, 223; relation of, with heat, 162, 245 et seq.; amount required to develop electricity, 131 et seq.; produces various physical changes, 139; substantial conception of, 183-184. See _Energy_. World, the, what it consists of, 208. World-particles, 203. Wronsky, 172. Wundt, on causality and the axioms of physics, 157-159; 359 footnote. Xenophon, 49, footnote. Young, Thomas, on energy, 173. Zelter, 35. Zeuner, 171. Zoölogy, comparison in, 239. THE WORKS OF ERNST MACH.