The vertical lines are hour lines and the horizontal lines show the force of the wind in miles an hour and also in pounds a square foot.] These disturbances give rise to the very marked fluctuations in the force of the wind known as gusts. There are certain forms of anemometer especially designed to record the gustiness of the wind. A record of the wind’s force is traced by a pen on a moving strip of paper, and the “anemogram” thus obtained shows a continuous series of irregularities, the extent of which increases with the strength of the wind. The puffs and lulls often alternate at intervals of a few seconds or less, and the actual force of the wind at a given instant may be many times greater than its average force for, say, five minutes. An ordinary anemometer does not indicate these rapid fluctuations, but merely shows the time required for a mile of wind to flow past the instrument. Thus when such an instrument tells us that the wind is blowing at the rate of 40 miles an hour, it may actually be varying between 20 and 60 miles an hour, or between even wider limits. Since the matter became of practical importance on account of the needs of aviation, many interesting studies have been made of the effects of different kinds of topography upon the overlying air currents. A striking example of the eccentric winds that sometimes prevail in mountain valleys has been described by Mr. B. M. Varney, of the University of California, in the “Monthly Weather Review.” From the summit of a steep cliff about 1,100 feet above the floor of Yosemite Valley the writer launched broad sheets of tissue paper, and, with the aid of powerful binoculars, followed their flight as they were carried in huge spirals, thousands of feet in diameter, finally disappearing beyond the mountains on the opposite side of the valley. The accompanying sketch shows the path of one of these papers. From its starting point at _A_ until it passed behind the summit of Liberty Cap (_B_), more than a mile distant, the paper was watched for 7 minutes. The top of Liberty Cap is some 1,600 feet above the point at which the flight began. This sketch visualizes one of the ticklish problems that will some day confront the pilot of a sight-seeing or mail-carrying aeroplane in the Yosemite National Park. [Illustration: AIR CURRENTS IN YOSEMITE VALLEY (_Sketched by B. M. Varney._) The flight of a sheet of paper across the valley.]