NIEVE PENITENTE IN THE ARGENTINE ANDES 117 CLOUDS OR FOG CASCADING THROUGH LAST FORK CAÑON INTO SANTA ANITA CAÑON 136 CLOUDBURST IN SOUTHERN UTAH 137 PHOTOGRAPHIC ANALYSIS OF LIGHTNING--SINGLE AND DOUBLE DISCHARGES 160 PHOTOGRAPH OF LIGHTNING, SHOWING “BLACK” FLASHES 161 ATMOSPHERIC ELECTRICITY INSTRUMENTS 161 GLASS WEATHER MAP IN THE UNITED STATES WEATHER BUREAU 224 THE SUN DRAWING WATER 225 ORCHARD HEATERS IN OPERATION AS PROTECTION AGAINST EXPECTED FROST 256 SNOW SURVEYOR AT WORK, USING A CYLINDRICAL SNOW SAMPLER 257 SNOW ROLLERS, OR WIND-BLOWN SNOWBALLS 257 BED OF THE POTOMAC RIVER PHOTOGRAPHED FROM THE AIR 288 DRILLING WITH COMPRESSED AIR IN A COPPER MINE 289 LAUNCHING A WEATHER BUREAU KITE TO EXPLORE THE AIR OVER THE OCEAN 289 WEATHER BUREAU KIOSK, UNION SQUARE, SAN FRANCISCO 320 UNITED STATES WEATHER BUREAU STATION AT PEORIA, ILLINOIS 320 CENTRAL OFFICE OF THE UNITED STATES WEATHER BUREAU AT WASHINGTON 321 CHAPTER I THE ANATOMY OF THE ATMOSPHERE Two quite different conceptions of the substance called “air” are current in the world. One has prevailed from time immemorial. The other is wholly modern. One is the popular view, the other the scientific. Ancient philosophers regarded air as one of the four “elements” of which all things were supposed to be made. Average humanity, though it did not concern itself with philosophy, must have begun, almost as soon as it realized the existence of air at all, to think of it as something that, however it changed its state from hot to cold, dry to moist, pure to impure, was fundamentally uniform--a single entity. Certainly this idea is in full vigor today. The air that we breathe, supply to our fires, stir with fans, pump into bicycle tires, fly in--the air that asserts its independence of our will in the wind and the weather--gives us the impression of individuality. We instinctively rank it with water among the simple, definite things in the repertory of nature.