There is an immense difference of opinion again as regards skin friction. Some writers believe the air has a tendency to stick to certain materials more than to others, and that this difference is so great as to materially retard some machines in their passage through the air. A complicate series of tables has been worked out in great detail to show the exact amount of this friction on various bodies. Some aviators go to great pains to make every part of their aeroplanes as slippery as possible. This is done by polishing the surfaces exposed to the air and in some cases enclosing the foreward part of the aeroplane, like a ship’s prow, to diminish friction. On the other hand we find some of the greatest authorities on aviation disregarding this question almost entirely. In the Wright machines, for instance, the surface of the wings is usually left comparatively rough, and the sticks and wires are placed without any attention to diminishing friction. This is true as well of the Delegrange, Voison and Farman machines. Still other aviators design every detail of their machines to cut down this so-called skin friction. The uprights, for instance, are made elliptical in shape, with the sharp edges turned forward so that they will cut their way the more smoothly through the air. Several interesting attempts have been made to design a prow for an aeroplane which will cut the air with the least possible amount of friction. It is noticeable in these designs that the prows are very blunt, like the prow of a canal boat, and not as might be expected, sharp and narrow like that of a racing craft. The blunt-nosed prow is considered best for the air ship, because the air being one eight hundredth as dense as water offers very little resistance to its entrance. It is so much easier, in other words, to push an air ship through the air than a boat through water that there is no object in sharpening its nose. The air, however, in flowing along the sides of a rapidly moving body sticks to it, and retards its progress relatively more than water retards a boat. It is important, therefore, that the body of the aeroplane be made very short, so that the sides will offer as little friction as possible. This reverses the proportions of a water-borne ship. We are so accustomed to see fast boats with long, narrow hulls, that it comes as a surprise to find that a fast air ship must have a very broad beam and as short a hull as possible.