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