Fig. 26.—Top view, showing how streams of air divide.
Other rising and falling currents are caused by the sun passing behind clouds. Portions of the atmosphere are thus chilled and commence to fall while others upon which the sun is reappearing are heated and rise. Balloonists constantly encounter these changes in temperature and the gas in the bag expands or contracts so rapidly that it often requires a skillful pilot to prevent disaster.
These rising and falling currents caused by changes of temperature may be clearly seen on the surface of a lake if the observer is stationed at a height where he may look down on the water. In some places the water is covered with smooth glassy streaks which run in various directions. These smooth streaks are evidence of rising currents of air at those places. The rough spots which suddenly spread out and run across the water are caused by descending currents.
Therefore it is not good judgment to attempt gliding over ground broken by trees or other natural objects or when the wind is blowing over 12-15 miles per hour.
Fig. 27.—Showing how air currents pass over objects.
Do not under any consideration jump off from a height which rises prominently from surrounding objects. Otto Lilienthal, the brilliant German investigator and engineer who made over two thousand gliding flights specifically warned experimenters against starting glides from precipitous cliffs or buildings. There are two excellent reasons for this. First, because when jumping from such an elevation, a gust of wind rebounds from the sides and strikes the machine so that it requires great skill to counteract its influence. Second, because, the operator and machine are suddenly suspended high in the air.
Be satisfied at first by running against the wind on level ground and making short jumps. After some practice, operations may be transferred to a gentle slope and the length of the glides considerably increased. If the experimenter thus proceeds slowly without impatience, there is no danger in gliding. It is said that the Wright brothers never so much as turned an ankle in the hundreds of flights they made, before building a power driven machine.
