The ocean of air in which human beings live, even as deep-sea fishes live at the bottom of the liquid ocean, is called the _atmosphere_. Unlike the liquid ocean, it diminishes rapidly in density from the bottom upward. At an altitude of three and one-half miles it is only half as dense as at sea-level. This is higher than the highest permanent habitations of man. Mountain-climbers and balloonists have attained greater altitudes; but above a level of about five miles the air is too greatly rarefied to support life. Balloonists who ascend still higher must carry a supply of oxygen with them. A little above the ten-mile level the air is only one-eighth as dense as at sea-level. The atmosphere extends at least 300 miles above the earth, at which height its density is computed to be only one two-millionth as great as at sea-level. The weather with which human beings are concerned may be said to extend upward seven or eight miles; _i.e._, to the level of the higher clouds. The layer of the atmosphere lying between sea-level and the upper cloud level has certain characteristics that distinguish it from the air above it, and is known as the _troposphere_. The heating of the atmosphere by the sun is the beginning of all weather, and the temperature of the air is the most important weather element. As soon as we begin to study atmospheric temperature, we encounter a paradox. The heat of the air is all derived from the sun (except a minute quantity from the interior of the earth, and an infinitesimal quantity from other heavenly bodies), and it would therefore seem at first glance that the upper layers of the atmosphere should be warmer than the lower. Experience proves the reverse to be the case. A mountain overgrown with tropical vegetation on its lower slopes is, if high enough, crowned with eternal snows. A thermometer carried upward in the air shows under average conditions a fall of temperature of one degree (Fahrenheit) for every 300 feet of ascent. This fall of temperature with ascent continues to the upper limit of the troposphere, where the average temperature is something like 70 degrees below zero. [Illustration: THE NEW IDEA IN WEATHER OBSERVATORIES The Observatory of the Ebro (Spain), founded by Spanish Jesuits, is devoted to studying the interrelations of sun, earth and air. Its admirable equipment includes apparatus for the direct and spectroscopic study of the sun, for measuring solar radiation, atmospheric electricity, earth currents, terrestrial magnetism, and earthquakes; besides the ordinary routine of a meteorological observatory. The results of all these observations are published side by side, to facilitate comparison.]