CHAPTER XXII.

Evaporation—Distribution of Vapour—Dew—Hoar-Frost—Fog—Region of Clouds—Forms of Clouds—Rain—Distribution of Rain—Quantity—Number of rainy Days in different Latitudes—Rainless Districts—Snow Crystals—Line of perpetual Snow—Limit of Winter Snow on the Plains—Sleet—Hail—Minuteness of the ultimate Particles of Matter— Their Densities and Forms—Their Action on Light—Colour of Bodies— Colour of the Atmosphere—Its Absorption and Reflection of Light— Mirage—Fog Images—Coronæ and Halos—The Rainbow—Iris in Dewdrops— The Polarization of the Atmosphere—Atmospheric Electricity—Its Variations—Electricity of Fogs and Rain—Inductive Action of the Earth—Lightning—Thunder—Distribution of Thunder-Storms—Back Stroke—St. Elmo’s Fire—Phosphorescence—Aurora—Magnetism— Terrestrial Magnetism—The Dip—Magnetic Poles and Equator—Magnetic Intensity—Dynamic Equator—Declination—Magnetic Meridian—Lines of equal Variation—Horary Variations—Line of Alternate Horary Phenomena—Magnetic Storms—Coincidence of the Lines of equal Magnetic Intensity with Mountain Chains—Diamagnetism 272