Another fact very important to the aeronaut is that the air is in _constant motion_. Owing to its ready expansion by heat, a body of air occupying one cubic foot when at a temperature of 32° F. will occupy more space at a higher temperature, and less space at a lower temperature. Hence, heated air will flow upward until it reaches a point where the natural density of the atmosphere is the same as its expanded density due to the heating. Here another complication comes into play, for ascending air is cooled at the rate of one degree for every 183 feet it rises; and as it cools it grows denser, and the speed of its ascension is thus gradually checked. After passing an altitude of 1,000 feet the decrease in temperature is one degree for each 320 feet of ascent. In general, it may be stated that air is expanded one-tenth of its volume for each 50° F. that its temperature is raised. This highly unstable condition under ordinary changes of temperature causes continual movements in the air, as different portions of it are constantly seeking that position in the atmosphere where their density at that moment balances the earth’s attraction. Sir Hiram Maxim relates an incident which aptly illustrates the effect of change of temperature upon the air. He says: “On one occasion, many years ago, I was present when a bonded warehouse in New York containing 10,000 barrels of alcohol was burned.... I walked completely around the fire, and found things just as I expected. The wind was blowing a perfect hurricane through every street in the direction of the fire, although it was a dead calm everywhere else; the flames mounted straight in the air to an enormous height, and took with them a large amount of burning wood. When I was fully 500 feet from the fire, a piece of partly burned one-inch board, about 8 inches wide and 4 feet long, fell through the air and landed near me. This board had evidently been taken up to a great height by the tremendous uprush of air caused by the burning alcohol.” That which happened on a small scale, with a violent change of temperature, in the case of the alcohol fire, is taking place on a larger scale, with milder changes in temperature, all over the world. The heating by the sun in one locality causes an expansion of air at that place, and cooler, denser air rushes in to fill the partial vacuum. In this way winds are produced. So the air in which we are to fly is in a state of constant motion, which may be likened to the rush and swirl of water in the rapids of a mountain torrent. The tremendous difference is that the perils of the water are in plain sight of the navigator, and may be guarded against, while those of the air are wholly invisible, and must be met as they occur, without a moment’s warning. [Illustration: