The rubber strands should not be allowed to come in contact with any metal parts of the model. The copper that is often used for wiring is especially injurious and tends to decompose the rubber. The hooks of both the propeller and motor anchorage should be covered with a piece of rubber tubing. This serves a double purpose. With this protection, the rubber when tightly twisted is in no danger of being cut by the wire or of taking up the oxides which quickly eat through it. [Illustration: A notable model possessing unusual stability. Built by W.S. Howell, Jr.] [Illustration: Front view of model built by W.S. Howell, Jr.] It requires an expert to pick out the best quality of rubber. If the strands be examined under a magnifying glass, it will be found that the edges of fresh rubber of the best quality are clean-cut, whereas the cheaper rubber, and that which is worn, has commenced to granulate, giving the edges a ragged appearance. A simple test is to stretch the rubber over a ruler. A good rubber, in first-class condition, will stretch about seven times its length, and on being released instantly spring back to its original size. The same rubber should stretch to ten times its length without breaking. There is a great difference of opinion among the most successful model builders as to the best shape of rubber strands. Some prefer the flat, band rubber, while others are obtaining satisfactory results with rubber cut in square strands. The strand used by the English model builders is seldom more than one-sixteenth of an inch square, while in America one-eighth of an inch strand is commonly used. Experiments have been made with a single strand of rubber one-fourth of an inch square, but the results have not been satisfactory. One theory is that the corners of the square rubber tend to cut into one another and quickly wear out, and that a perfectly round strand would be the more efficient. At present there are no such strands on the market. It is argued by some that the square strand in twisting must be turned on itself further than the flat strand, and is therefore placed under an unnecessary strain. After all, the advantage of one form over another is fractional, and an extra strand added to the motor will balance any possible defects.