l.U THE EARTH AS MAN'S ABODE of the atmosphere and man breathes oxygen to burn these foods in his body to furnish energy. The weight of the atmosphere is about 5,000,000,000,000,000 tons, or 30,000,000 tons per square mile. In temperate regions on the average of at least 100,000 tons of water \apor are present over each square mile of the earth's surface. In the tropics several times this amount of water vapor may be present, because warm air can hold much more water vapor than cool air. At high altitudes mountains such as Mount Everest are always very cold. At thirteen miles above sea level the temperature is 80° F. below zero. At this height there is very little matter to absorb heat. At this altitude the atmosphere is so rarefied that no breathing organisms could survive unless they took a supply of compressed air them. or oxygen with Ninety-five per cent of the atmosphere is contained in the layer within thirteen miles of sea level. The Atmosphere May Once Have Had a Different Composition. It is quite possible that all of the water on the earth was once contained in the atmosphere at a much higher temperature than that which now exists on the earth's surface. Perhaps the atmosphere likewise contained much more carbon dioxide and less oxygen than it does today. Plants build their body structures largely out of the carbon dioxide that they take from the air and the water and its dissolved salts taken from the soil. By this process oxygen is added to the atmosphere. On the other hand, animals use oxygen from the air to burn the plants which they eat to produce heat and other forms of energy. Carbon dioxide is added to the air by the process of respiration, as well as by the decay of plant and animal tissues and the burning of fuels. It is removed from the air, on the other hand, by its reaction with substances in water solution to form vast deposits of carbonates such as limestone and marble. Today these processes seem to have reached a condition of equilibrium. At least so we trust, for a very small decrease in the carbon dioxide content of the air would result in the extinction of all plant life. Whole mountain ranges of carbonates exist ; and tremendous deposits represent huge amounts (at least 30,000 times as of peat, coal, and oil much as in the air today) of carbon which must have been present at one time or another in the earth's atmosphere. The very luxuriant plant growth which was responsible for these coal deposits may be attributed to the higher temperatures and higher carbon dioxide content of the atmosphere at that time. Undoubtedly plant growth added a great deal of oxygen to the air during these periods.
THE NATURE OF THE EARTH 135 The Hydrosphere Has Been Explored Too. The liquid sphere of the earth is extraordinary in that it may evaporate and become a part of the atmosphere or freeze and become a part of the Hthosphere. Not only that, it can also dissolve some of the atmosphere and thus support marine life, and it has dissolved so much of the lithosphere solid matter, in that ocean water now contains about 3^ per cent solution consisting largely of chlorides and sulfates of sodium, magnesium, and potassium. The water portion of the earth is spoken of as the hydrosphere. The pressure of water increases one atmosphere, i.e., 14.7 pounds, for every 33 feet of depth. At a depth of one mile the pressure is over a ton per square inch. Man cannot go to depths greater than 300 feet safely with diving suits, but William Beebe and Otis Barton descended more than a half-mile into the ocean in a strong steel sphere with fused quartz windows, called a bathysphere. William Beebe's bathysphere developed a leak in a preliminary test in which it was let down into the ocean empty. The tremendous pressure of the ocean depths can be imagined from the following account which Beebe recorded in his book. Half Mile Down.^ I began to unscrew the giant wing bolt in the center of the door and, after the first few turns, a strange high singing came forth, then a fine mist, steam-Hke in consistency, shot out. ... I cleared the deck in front of the door of everyone, staff and crew. . . . Carefully, little by little, two of us turned the brass handles. . . . Suddenly, without the slightest warning, the bolt was torn from our hands, and the mass of heavy metal was shot across the deck like a shell from a gun. The trajectory was almost straight and the brass bolt hurtled into the steel winch thirty feet away across the deck and sheared a half-inch notch gouged out by the harder metal. This was followed by a solid cylinder of water, which slackened after a while to a cataract, pouring out of the hole in the door, some air mingling with the water, looking like hot steam, instead of compressed air shooting through ice-cold water. If I had been in the way, I would have been decapitated. Greater depths can be explored only by the indirect means of depthsounding devices and dragnets which bring to the surface many odd deep-sea fish. Below 3000 feet the ocean is dark as night, and many of these deep-sea fish have amazing lighting systems of their own. The greatest ocean depths so far recorded are: Feet . 28,680 Mundanae, near the Philippine Islands . . . 35,400 South Pacific Ocean 30,930 Milwaukee Deep in the North Atlantic Ocean Southern Atlantic 26,575 Indian Ocean 22,968 ' Harcourt, Brace and Company, 1934, pp. 153-154,