PLATE XII. THE 110–TON ARMSTRONG GUN 202 PLATE XIII. THE FORTH BRIDGE 292 PLATE XIV. THE TOWER BRIDGE IN COURSE OF CONSTRUCTION 298 PLATE XV. THE BROOKLYN BRIDGE 304 PLATE XVI. THE NORTH MOUTH OF THE GREAT TUNNEL, ST. GOTHARD RAILWAY 374 PLATE XVII. SPECTRA (Coloured Plate) 422 [Illustration: _Wind, Steam, and Speed_ (after TURNER). ] INTRODUCTION. Only by knowledge of Nature’s laws can man subjugate her powers and appropriate her materials for his own purposes. The whole history of arts and inventions is a continued comment on this text; and since the knowledge can be obtained only by observation of Nature, it follows that Science, which is the exact and orderly summing-up of the results of such observation, must powerfully contribute to the well-being and progress of mankind. Some of the services which have been rendered by science in promoting human welfare are thus enumerated by an eloquent writer: “It has lengthened life; it has mitigated pain; it has extinguished diseases; it has increased the fertility of the soil; it has given new securities to the mariner; it has furnished new arms to the warrior; it has spanned great rivers and estuaries with bridges of form unknown to our fathers; it has guided the thunderbolt innocuously from heaven to earth; it has lighted up the night with the splendour of the day; it has extended the range of the human vision; it has multiplied the power of the human muscles; it has accelerated motion; it has annihilated distance; it has facilitated intercourse, correspondence, all friendly offices, all dispatch of business; it has enabled man to descend to the depths of the sea, to soar into the air, to penetrate securely into the noxious recesses of the earth, to traverse the land in cars which whirl along without horses, to cross the ocean in ships which run ten knots an hour against the wind. These are but a part of its fruits, and of its first-fruits; for it is a philosophy which never rests, which has never attained, which is never perfect. Its law is progress. A point which yesterday was invisible is its goal to-day, and will be its starting-point tomorrow.”—(MACAULAY).