Hoarfrost (hōr´frŏst).—The white particles formed by the congelation of dew; white frost.
Hydrography (hī-drŏg´rȧ-fy̆).—The art of measuring and describing the sea, lakes, rivers, and other waters, with their phenomena.
Hygrometer (hī-grŏm´ê-tẽr).—An instrument for measuring the degree of moisture of the atmosphere.
Ignis fatuus (ĭg´-nĭs făt´ûŭs).—A phosphorescent light that appears, in the night, over marshy grounds, supposed to be occasioned by the decomposition of animal or vegetable substances, or by some inflammable gas,—popularly called also Will-with-the-wisp, or Will-o’-the-wisp, and Jack-with-a-lantern, or Jack-o’-lantern.
Ichthyosaurus (ĭk-thē-ō-saw´rus).—A large marine reptile, known only by fossil vertebræ and other bones, found in oolite rocks.
Labyrinthodon (lab-i-rin´thō-don), or Mastodon. A large animal, belonging to Amphibia, remains of which are found in Upper Trias rocks and strata.
Lepidodendron (lep-i-dō-den´dron).—Coal-plants, belonging to the Lycopods, of which very many remains are found in coal.
Lepidosteus (lep-i-dŏs´te-us).—Bony-pike fish, the fossil remains of which are found in rocks and earth strata.
Lightning (līt´nĭng).—A discharge of atmospheric electricity, accompanied by a vivid flash of light, commonly from one cloud to another, sometimes from a cloud to the earth. The sound produced by the electricity in passing rapidly through the atmosphere constitutes thunder.