When a flat surface like the side of a house is exposed to the breeze, the velocity of the wind exerts a force or pressure directly against the surface. This principle is taken into account in the design of buildings, bridges, and other structures. The pressure exerted per square foot of surface is equal (approximately) to the square of the wind velocity in miles per hour, divided by 300. Thus, if the wind velocity is thirty miles, the pressure against a house wall on which it acts directly is 30 × 30 ÷ 300 = 3 pounds per square foot: if the wind velocity is sixty miles, the pressure is 60 × 60 ÷ 300 = 12 pounds: if the velocity is ninety miles, the pressure is 90 × 90 ÷ 300 = 27 pounds, and so on. [Illustration: PRESSURE OF THE WIND] If the wind blows obliquely toward the surface, instead of directly, the pressure at any given velocity is reduced, but may still be considerable. Thus, in the sketch, let _ab_ represent a wall, toward which we are looking downward, and let the arrow _V_ represent the direction of the wind. The air particles will follow some such paths as those indicated, being deflected so as to finally escape around the ends of the wall. The result is that a pressure is produced which may be considered to act along the dotted line _P_, perpendicular to the wall. This is the invariable law: that no matter how oblique the surface may be, with reference to the direction of the wind, there is always a pressure produced against the surface by the wind, and this pressure always acts _in a direction perpendicular to the surface_. The amount of pressure will depend upon the wind velocity and the obliquity or inclination of the surface (_ab_) with the wind (_V_). Now let us consider a kite--the "immediate ancestor" of the aeroplane. The surface _ab_ is that of the kite itself, held by its string _cd_. We are standing at one side and looking at the _edge_ of the kite. The wind is moving horizontally against the face of the kite, and produces a pressure _P_ directly against the latter. The pressure tends both to move it toward the left and to lift it. If the tendency to move toward the left be overcome by the string, then the tendency toward lifting may be offset--and in practice _is_ offset--by the weight of the kite and tail. [Illustration: FORCES ACTING ON A KITE]