[If you have drawn the picture, Fig. 110, in advance, merely indicate the parts as you proceed; otherwise, point them out as you finish each part of the machine.] "This style of machine is known as the biplane, or two-plane. This upper part is one of the planes, and this lower part is the other. This part out in front is that portion of the steering apparatus which enables the aviator to guide the machine up or down, and this part at the back is to govern the side-to-side movements. When the machine stands on the ground it rests on these three little wheels, which are like bicycle wheels. Here sits the aviator, and directly back of him is the powerful little engine which sets the propeller whirling at the rear. The machine makes a noise like a swift-running motor boat or a motorcycle. It starts off on its wheels and rapidly increases its speed until it rises from the ground and sails away gracefully into the upper air. [Your drawing of Fig. 110 should now be complete.] "As you look at this machine, remember that it is not at all like a balloon. The bag of a balloon, filled with gas, is lighter than the air; hence, it stays up without any trouble, unless the bag breaks and lets the gas out. But the aeroplane has no gas bag; it is heavier than the air and it must '_keep a-goin'_' in order to stay up at all. Remember this: _Just as soon as the aeroplane stops, it comes crashing to the earth_, like so many have done, bringing death and destruction. [Quickly detach your drawing paper from your board, turn it one-fourth around and re-attach it with thumb tacks. With broad strokes of black crayon indicate the foreground. Add lines of mountains, completing Fig. 111.] [Illustration: Fig. 111] "You boys know how it is when you are riding a bicycle. Your wheel will stay upright as long as you are pushing ahead, but as soon as you stop the wheel topples over. "Sometimes the aeroplane engine fails to work, sometimes a wire or rod breaks, sometimes the aviator attempts to do some fancy flying which throws the machine out of balance, sometimes the wind prevents the machine from going on in its course. Any of these things may cause the machine to stop going forward and come dashing downward.