The life of your rubber motor may be lengthened by careful winding. As long as you wind up by turning the propeller by hand, it is safe to turn it as fast as you can, since the process is slow at best. In case the turning is done with a machine drill or some similar geared wheel, there is danger at some points of winding too fast. It is safe to wind as quickly as you can until the first row of knots has formed in the rubber strands, but at the moment the double strands begin to appear the winding should proceed very slowly. You will find that if you wind very quickly the double knots will appear in tight groups or bunches, and that only after considerable winding do these begin to spread out evenly. This puts the rubber under a severe and unnecessary strain and shortens its life. The simplest way of locking the propellers when once wound up is to thrust a piece of cane or reed through the hooks. The twist of the motor will hold it tightly in position, so that you can carry your model about, even shake it vigorously without danger of dislodging it. If you have twin propellers, use a strip long enough to pass through both hooks. Remove the strip just before starting. Be careful, of course, that your axles have not been thrown out of plumb. It will be found very convenient to equip your model with a single clasp for holding the propellers after they have been wound up, which may be easily released. It is awkward to keep them from slipping. An effective break may be made by attaching two strips of reed or cane, such as you use for skids, to either side of the motor base, so that the free ends will pass between the propeller blades and the frame, thus locking them fast. These bands should spring outward and be held in position by rubber bands running from one to the other. To release the propellers, simply pinch the two free ends together, and the propellers will be freed at the same instant. Do not keep your motor wound up a moment longer than you can help. It is very trying to the rubber to be held in this tightly-twisted position. In mounting your propeller, it is well to make the support for the bearing of the propeller axle as long as possible. An axle turning in a shaft one inch in length will meet with much less resistance than in a half-inch shaft, and with a good motor an inch-and-a-half shaft is still better. The rear stick of your motor base, which often holds the propeller axle, is usually made as thin as possible and rarely gives you more than a half-inch support. It is a good plan to lengthen the shaft by attaching a block of wood to the frame and passing the axle through it. Cut from a strip one-half an inch square a piece one inch in length, or whatever seems necessary. This may be mortised slightly into the stick and glued at right angles. [Illustration: An excellent monoplane capable of long flights.] [Illustration: Long-distance model built by Percy Pierce.]