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The atmosphere is the vast ocean of air that envelops the earth and makes life possible on our globe. It absorbs the heat and vapors caused by the action of the sun upon the surface of both land and water, and is the medium through which the ever-changing phenomena of climate and weather are produced. The two great forces of nature acting in connection with it are gravitation and heat, or solar radiation; and the results of their ceaseless action may be summed up as follows: (1) Temperature, or heat, which we soon learn to know by our senses, and to measure by the thermometer. (2) Evaporation, which changes the weight of the air by carrying invisible moisture through it. This change of weight is indicated by the barometer. (3) Condensation, producing fog, dew, rain, hail, and snow; all estimated accurately by the rain gauge or pluviometer. (4) Motions, as in the winds, varying from the gentle breeze to the awful cyclone, the force and velocity of which are indicated by the anemometer. (5) Electricity, producing lightning, thunder, magnetic and chemical changes in the atmosphere. (6) Optical Phenomena, such as rainbows, haloes, coronas, mirage, and the auroras.