For example, if the tail plane or flat, finlike surface that projects from the sides of the tail of the body, or fuselage, has too much “incidence,” or, in other words, is slanted at too sharp an angle downward, it has a tendency in flight to lift the rear of the machine and to make it dive. A seaplane, when properly constructed, is so evenly balanced that, when the crane that lifts it off the mother ship holds it suspended in the air, the machine is equipoised like a bird with wings spread in flight. If the plane is heavier on one side than on the other, it will, while “banking,” or turning a corner, slide toward the centre of the circle; that sometimes causes a “tail spin,” in which the machine whirls round as if it had been caught in a whirlpool. That is a very difficult situation, for an aviator usually ends in a smash at the bottom of the whirlpool unless the pilot has altitude enough to flatten out his plane before it gets too close to the ground. These things were all taught before the novice went up in the air. Map reading and air navigation were the next studies in military aviation schools. First, the student learned how to judge the height of hills and the size of towns from different altitudes, so that when flying he could tell what part of the country he was passing over. Many of the schools perched the prospective fliers high in the air in a classroom and spread out a miniature landscape made of dirt and sand on a map beneath them so they could get practice in perspective. Of course, when an aviator is lost in the fog or above the clouds he needs to use all the instruments on board to find his position. For that purpose drift instruments are mounted on aircraft; those tell how much the air-currents, which have the same effect on aircraft as the tide has on a boat, have driven him off his course. A compass indicates the direction in which he is travelling, and other instruments show him whether his machine is climbing, diving, or “banking”; the aneroid barometer indicates the altitude. It is essential, of course, for the aviator to know how to read those instruments correctly. Without the information they give him, he might not know, if flying at night or in a cloud, that his craft was climbing at a dangerous angle until wrenches or other loose implements began to fall out of the machine. As the next step in the training the student learns the controls. To do that he runs the “taxi” or “lawn-mower,” as the training-machine is called, up and down the field. The “hopping” of this machine familiarizes him with “getting off” and landing, and with the noise of the propeller. After he has learned to steer his machine in a straight line, he takes longer “hops” until he is thoroughly familiar with the “joy stick” which pulls the elevators or ailerons up or down or operates the rudder.