Nature and Character of Mineral Veins—Metalliferous Deposits— Mines—Their Drainage and Ventilation—Their Depth—Diffusion of the Metals—Gold—Silver—Lead—British Mines—Quicksilver—Copper—Tin— Cornish Mines—Coal—Iron—Most abundant in the Temperate Zones, especially in the Northern—European and British Iron and Coal— American Iron and Coal—Arsenic and other Metals—Salt—Sulphur— Diffusion of the Gems 168 CHAPTER XVI. The Ocean—Its Size, Colour, Pressure, and Saltness—Tides—Waves— their Height and Force—Currents—their Effect on Voyages— Temperature—The Stratum of Constant Temperature—Line of Maximum Temperature—North and South Polar Ice—Inland Seas 188 CHAPTER XVII. Springs—Basins of the Ocean—Origin, Course, and Heads of Rivers— Hydraulic Systems of Europe—African Rivers—the Nile, Niger, &c. 209 CHAPTER XVIII. Asiatic Rivers—Euphrates and Tigris—River Systems South of the Himalaya—Chinese Rivers—Siberian Rivers 224 CHAPTER XIX. River Systems of North America—Rivers of Central America—Rivers of South America and of Australia 234 CHAPTER XX. Lakes—Northern System of the Great Continent—Mountain System of the same—American Lakes 245 CHAPTER XXI. Temperature of the earth—Temperature of the Air—Radiation—Foci of Maximum Cold—Thermal Equator—Its Temperature, mean and absolute— Isothermal Lines—Continental and Insular Climates—Extreme Climates—Stability of Climate—Decrease of Heat in Altitude—Line of Perpetual Snow—Density of the Atmosphere—The Barometer— Measurement of Heights—Variations in Density and their Causes— Horary Variations—Independent Effect of the dry and aqueous Atmospheres—Mean height of Barometer in different Latitudes— Depression in the Antarctic Ocean and in Eastern Siberia— Barometric Storms—Polar and Equatorial Currents of Air— Trade-Winds—Monsoons—Land and Sea Breezes—Gyration of the Winds in the Extra-Tropical Zones—Winds in Middle European Latitudes— Hurricanes—The Laws of their Motion—Their Effect on the Barometer—How to steer clear of them—The Storm-Wave— Storm-Currents—Arched Squalls—Tornadoes—Whirlwinds—Water Spouts 254 CHAPTER XXII.