thermodynamic laws, 189-90; transmission through bodies, 176-9; transmission through space, 180-4; units, iv, 154, 189-90, vii, 369, viii, 374, x, 269; universal presence, v, 345; vacuums, v, 345-58; wastage in engines, v, 155, 161, 165-6, 351 (see also Heat Waves, Temperature) Heat Engines, iv, 192, 193-4, xvi, 135 Heat Equator, xiv, 347 Heating, dynamic, i, 90 Heating Systems, iv, 185-7; dryness from, xiv, 353; water advantages in, iv, 162 Heat Lightning, i, 148, vii, 205, 213 Heat Prostrations, ix, 316 Heat Regulators, vii, 87-8 Heat Stroke, x, 251-2, 274 Heat Thunderstorms, i, 138, 151, vii, 217 Heat Waves, vi, 119, 269, 270, vii, 371; length and frequency, 260; transmission of, iv, 180-4; volcanic dust effects, i, 59 Heath Family, xiii, 202; shrubs of, 274 Heavier-than-air Machines, v, 230-8; principles, i, 286-9; remarks on, vii, 76 Heberden, William, x, 104 Hebrew Language, xv, 162; religious words from, 161 Hebrews, hemp fiber unknown to, xiii, 239; ideas of insanity, x, 356; unclean animals of, xii, 311 Hedgehogs, xii, 366, 367 Heidelberg Man, xv, 92, 93-5; period of, 102 Heidelberg Race, xv, 96-7 Height, human, at morning and night, ix, 65; rate of growth, 32; of various races, xv, 38-9 Heights, oceanic, xiv, 286 Helicopter, i, 42 Heligoland, coast destruction, iii, 56 Heliocentric System, ii, 43-4; known in Egypt, xvi, 69; taught by Aristarchus, ii, 28 (see also Copernican System) Heliometer, ii, 311 Helioscope, ii, 172-3 Heliotaxis, xi, 52-3, 61 Heliotropism, in hydroids, xii, 34 Helium, atmospheric, i, 11, 12, ii, 232; boiling and freezing points, iv, 173; critical temperature, 173; density of, 113; discovery, i, 12, viii, 302, xvi, 194; frozen, v, 345; liquefaction, i, 32, xvi, 194; liquefaction temperature, v, 348; molecular velocity in, iv, 133, viii, 185, 186; monatomic, viii, 309; production by disintegration, i, 12; specific heat ratio for, iv, 156; symbol and atomic weight, viii, 383; use of, in balloons, iv, 108 Hellbenders, xii, 171-2 Hell Gate, tidal race of, xiv, 294 Hellgrammite, xii, 106 Helmets, modern, xv, 221 Helmholtz, chemical work, xvi, 142; "Conservation of Energy," 181-2; contraction theory, ii, 380; medical work of, x, 131; on Young, 97; pupil of Muller, 118, 128; sound studies, iv, 52, 233; theory of life, xii, 9; thermodynamic studies, xvi, 136; theory of color vision, x, 96 Heloderma, xii, 207 Hematite, iii, 332, 356, 358, viii, 47, 156 Hemlocks, in class of conifers, xiii, 174; foliage, 270-1; planting conditions, 270; poison, 250; roots, 17; in northern forests, xiv, 372 Hemoglobin, ix, 181-3, 184, 258-9, 275, x, 337; deficiency in anemia, 337