Atmosphere

—The air in which we live and which we breathe, whose weight pre es on our bodies internally and externally and so is not perceived. Besides oxygen, nitrogen, argon, etc., there is present in the atmosphere about 0.04% by volume, of carbon dioxide, a variable amount of aqueous vapor, ammonia in various forms, and solid matter or dust. The pre ure of the atmosphere at the sea level is about 14.7 pounds, per square inch; for a rough approximation it may be a umed that the pre ure decreases 4 pound per square inch for every 1000 ft. of ascent.