If the weight _W_ of the aeroplane acts downward at the center of the wing (at _o_ in the accompanying sketch), while the direct pressure _P_ acts at some point _c_ farther along toward the upper edge of the wing, the two forces _W_ and _P_ tend to revolve the whole wing in the direction indicated by the curved arrow. This rotation, in an aeroplane, is resisted by the use of a tail plane or planes, such as _mn_. The velocity produces a direct pressure _P´_ on the tail plane, which opposes, like a lever, any rotation due to the action of _P_. It may be considered a matter of rather nice calculation to get the area and location of the tail plane just right: but we must remember that the amount of pressure _P´_ can be greatly varied by changing the inclination of the surface _mn_. This change of inclination is effected by the operator, who has access to wires which are attached to the pivoted tail plane. It is of course permissible to place the tail plane _in front_ of the main planes--as in the original Wright machine illustrated: but in this case, with the relative positions of _W_ and _P_ already shown, the forward edge of the tail plane would have to be depressed instead of elevated. The illustration shows the tail built as a biplane, just as are the principal wings (page 141). Suppose the machine to be started with the tail plane in a horizontal position. As its speed increases, it rises and at the same time (if the weight is suspended from the center of the main planes) tilts backward. The tilting can be stopped by swinging the tail plane on its pivot so as to oppose the rotative tendency. If this control is not carried too far, the main planes will be allowed to maintain some of their excessive inclination and ascent will continue. When the desired altitude has been attained, the inclination of the main planes will, by further swinging of the tail plane, be reduced to the normal amount, at which the supporting power is precisely equal to the load; and the machine will be in vertical equilibrium: an equilibrium which demands at every moment, however, the attention of the operator. In many machines, ascent and tilting are separately controlled by using two sets of transverse planes, one set placed forward, and the other set aft, of the main planes. In any case, quick ascent can be produced only by an increase in the lifting force _L_ (see sketch, page 24) of the main planes: and this force is increased by enlarging the angle of inclination of the main planes, that is, by a controlled and partial tilting. The forward transverse wing which produces this tilting is therefore called the _elevating rudder_ or elevating plane. The rear transverse plane which checks the tilting and steadies the machine is often described as the _stabilizing plane_. _Descent_ is of course produced by _decreasing_ the angle of inclination of the main planes. [Illustration: ROE'S TRIPLANE AT WEMBLEY (From Brewer's _Art of Aviation_)] Steering