Something definite has been accomplished in the determination of wind velocities, though this varies largely with the locality. In the United States the average speed of the winds is 9½ miles per hour; in Europe, 10⅓ miles; in Southern Asia, 6½ miles; in the West Indies, 6⅕ miles; in England, 12 miles; over the North Atlantic Ocean, 29 miles per hour. Each of these average velocities varies with the time of year and time of day, and with the distance from the sea. The wind moves faster over water and flat, bare land than over hilly or forest-covered areas. Velocities increase as we go upward in the air, being at 1,600 feet twice what they are at 100 feet. Observations of the movements of cloud forms at the Blue Hill Observatory, near Boston, give the following results: +---------------+---------+---------------+ | Cloud Form | Height | Average Speed | | | in Feet | per Hour | +---------------+---------+---------------+ | Stratus | 1,676 | 19 miles. | | Cumulus | 5,326 | 24 miles. | | Alto-cumulus | 12,724 | 34 miles. | | Cirro-cumulus | 21,888 | 71 miles. | | Cirrus | 29,317 | 78 miles. | +---------------+---------+---------------+ In winter the speed of cirrus clouds may reach 96 miles per hour. There are forty-nine stations scattered over Germany where statistics concerning winds are gathered expressly for the use of aeronauts. At many of these stations records have been kept for twenty years. Dr. Richard Assman, director of the aerological observatory at Lindenburg, has prepared a comprehensive treatise of the statistics in possession of these stations, under the title of _Die Winde in Deutschland_. It shows for each station, and for each season of the year, how often the wind blows from each point of the compass; the average frequency of the several degrees of wind; when and where aerial voyages may safely be made; the probable drift of dirigibles, etc. It is interesting to note that Friedrichshafen, where Count Zeppelin’s great airship sheds are located, is not a favorable place for such vessels, having a yearly record of twenty-four stormy days, as compared with but two stormy days at Celle, four at Berlin, four at Cassel, and low records at several other points. In practical aviation, a controlling factor is the density of the air. We have seen that at an altitude of five miles the density is about three-eighths the density at sea-level. This means that the supporting power of the air at a five-mile elevation is so small that the area of the planes must be increased to more than 2½ times the area suited to flying near the ground, or that the speed must be largely increased. Therefore the adjustments necessary for rising at the lower level and journeying in the higher level are too large and complex to make flying at high altitudes practicable--leaving out of consideration the bitter cold of the upper regions.