In the case of winds, the inertia of the moving mass becomes what is commonly termed “wind pressure” against any object not moving with it at an equal speed. The following table gives the measurements of wind pressure, as recorded at the station on the Eiffel Tower, for differing velocities of wind: +----------+------------+---------------+ | Velocity | Velocity | Pressure | | in Miles | in Feet | in Pounds on | | per Hour | per Second | a Square Foot | +----------+------------+---------------+ | 2 | 2.9 | 0.012 | | 4 | 5.9 | 0.048 | | 6 | 8.8 | 0.108 | | 8 | 11.7 | 0.192 | | 10 | 14.7 | 0.300 | | 15 | 22.0 | 0.675 | | 20 | 29.4 | 1.200 | | 25 | 36.7 | 1.875 | | 30 | 44.0 | 2.700 | | 35 | 51.3 | 3.675 | | 40 | 58.7 | 4.800 | | 45 | 66.0 | 6.075 | | 50 | 73.4 | 7.500 | | 60 | 88.0 | 10.800 | | 70 | 102.7 | 14.700 | | 80 | 117.2 | 19.200 | | 90 | 132.0 | 24.300 | | 100 | 146.7 | 30.000 | +----------+------------+---------------+ In applying this table, the velocity to be considered is the net velocity of the movements of the airship and of the wind. If the ship is moving 20 miles an hour _against_ a head wind blowing 20 miles an hour, the net velocity of the wind will be 40 miles an hour, and the pressure 4.8 lbs. a square foot of surface presented. Therefore the airship will be standing still, so far as objects on the ground are concerned. If the ship is sailing 20 miles an hour _with_ the wind, which is blowing 20 miles an hour, the pressure per square foot will be only 1.2 lbs.; while as regards objects on the ground, the ship will be travelling 40 miles an hour. [Illustration: Apparatus for the study of the action of air in motion; a blower at the farther end of the great tube sends a “wind” of any desired velocity through it. Planes and propellers of various forms are thus tested.] Systematic study of the movements of the air currents has not been widespread, and has not progressed much beyond the gathering of statistics which may serve as useful data in testing existing theories or formulating new ones. It is already recognized that there are certain “tides” in the atmosphere, recurring twice daily in six-hour periods, as in the case of the ocean tides, and perhaps from the same causes. Other currents are produced by the earth’s rotation. Then there are the five-day oscillations noted by Eliot in India, and daily movements, more or less regular, due to the sun’s heat by day and the lack of it by night. The complexity of these motions makes scientific research extremely difficult.