Water, density of, compared to air, 15; infinitesimal size of molecule of, 30; union of hydrogen and oxygen to constitute, 32; and bacteria, 43; commercial and scientific unit of heat and, 50, 51; boiling point of, 58; boiling point of, as gauge for altitude, 60, 61; frost as affected by body of, 90, 91; rejects heat by reflection, 172; solar rays penetrate, 173; temperatures of large bodies of, 173; difference in freezing temperature of fresh and salt, 173; salt, better conductor of heat, 173; a wonderful phenomenon of fresh, 173-175; low temperature of, of ocean bottoms, 175, 176; temperature of, of inclosed seas and oceans, 176, 177; latitude, season and depth change temperature of, 177, 178; direction of wind affects shore temperature of, 178; has great capacity for heat, 200, 201 Water vapor, and earth’s atmosphere, 8; absorption of sun’s rays by, 8; level of, 8; one of earth’s atmospheres, 29; density of, 37; varies according to locality, 37, 38; temperature and, 38; precipitation of, 38, 231; transformations of, 38; and the dew point, 38; saturation point and temperature, 38; measured by hygrometer, 39; and frost, 85-97; protects earth from freezing, 170; changes in sun’s rays effected by, 170; a separate atmosphere, 231; and raindrops, 284; rainfall and instantaneous precipitation of all, 285; and snow, 285-287; and fog, 288 Waves, difference between light, heat and sound, 51; length of different, of solar energy, 51; velocity of, 51, 52 Weather, forecasting, with aneroid barometer, 74-79; moon has no influence on, 138-140; general rules for forecasting, 149-153; difference between climate and, 161; changes daily, 161; expectations of future forecasting of, 303, 304. _See_ also UNITED STATES WEATHER BUREAU “Weather Forecasting in the United States”, 151 Weather map, value of aviator in compiling, 23, 112-160; supplied by Weather Bureau, 112; value of, 112, 113; advantage of familiarity with, 113, 114; method of compiling, 114; collection of data for, 114, 115; marking isobars on, 115; Highs and Lows of, 115, 116; indication of storm action on, 115; arrows fly with wind on, 116, 117; winter storm of 1893 on, 117-123; temperature readings on, 119; indication of storm center on, 121; meaning of red lines on, 122, 123; forecasting tornadoes by use of, 144, 145; general rules for forecasting and the, 149-153; Prof. Henry compiles first, 296. _See_ also UNITED STATES WEATHER BUREAU Weather observers, voluntary, 66-79 Weather observations, from kites, 64; method of taking, 66-79; extent of area under, 155; practice of early meteorologists in, 155, 156; advantages enjoyed by the Weather Bureau in, 156-158. _See_ also UNITED STATES WEATHER BUREAU