The principles of energy are thus guides, not only to the scientific theorist, but to the practical engineer, and they have been established only through careful measurement. The simple observation of phenomena, and of the conditions under which they occur, could never have led to the establishment of such principles; and, though the carrying out of experiments which do not involve measurements is of great value, it is the careful measurement, however simple, which affords the highest training to the mind and hand, and without which any course of instruction in experimental physics is of little value. The Hindoos used to regard the earth as a vast dome carried on the backs of elephants. The elephants themselves, however, required support, and were represented as standing on the back of a gigantic tortoise. It does not, however, appear that any support was provided for the tortoise. In some respects this figure represents the apparently perpetual condition of scientific knowledge. Phenomena are investigated, and are shown to depend upon other actions which appear simpler or more fundamental than the phenomena at first observed. These, again, are found to obey laws which are of much wider application, or appear to be still more fundamental; but it may be that we are as far off as ever from discovering the great secret of the universe, the ultimate nature of all things. INDEX. A. Abbott, Faraday's letters to, 241, 246. Aberdeen University, Maxwell appointed professor in, 284; Young's report on, 203. Absorption, Rumford's experiments on, 185; of sun's rays by cloth of different colours, 99. Academy of Sciences, Franklin nominated Foreign Associate of, 111. Adjustment of the eye, Young's paper on the, 200. AEpinus's completion of Franklin's theory, 77. Air, Boyle's conception of the constitution of, 19. Air-pump, Boyle's experiments with, 19; constructed by Boyle, 27. American Independence, Declaration of, 113. American Philosophical Society, foundation of, 61. Ampere's theory, Faraday's views on, 257. Anchor-ring experiment, Faraday's, 260. Arago's experiment, 264. Argand lamp, efficiency of, 188. Armstrong gun, principle of the, 180. Atmospheric electricity, Faraday's experiments on, 254; obtained by a pointed rod, 84. Autobiography of Franklin, 39. Availability of energy, 326. B. Baily, Francis, repetition of the Cavendish experiment by, 146. Beats in music, explanation of, 209. Beggary in Bavaria banished by Rumford, 164. Bernoulli's, Daniel, molecular theory of gases, 299. Boston, blockade of, 110.