One of the most recent developments in aviation is the hydroplane, or water-plane as it is most commonly called. A hydroplane is an aeroplane fitted with floats instead of wheels, so that it will rise from, or alight upon, the surface of the water. Often water-planes have their floats removed and wheels affixed to the chassis, so that they may be used over land. From this you may think that the construction of a water-plane is quite a simple task; but such is not the case. The fitting of floats to an aeroplane has called for great skill on the part of the constructor, and many difficulties have had to be overcome. Those of you who have seen an acroplane rise from the ground know that the machine runs very quickly over the earth at a rapidly-increasing speed, until sufficient momentum is obtained for the machine to lift itself into the air. In the case of the water-plane the pilot has to glide or "taxi" by means of a float or floats over the waves until the machine acquires flying speed. Now the land resistance to the rubber-tired wheels is very small when compared with the water resistance to the floats, and the faster the craft goes the greater is the resistance. The great problem which the constructor has had to solve is to build a machine fitted with floats which will leave the water easily, which will preserve the lateral balance of the machine, and which will offer the minimum resistance in the air. A short flat-bottomed float, such as that known as the Fabre, is good at getting off from smooth water, but is frequently damaged when the sea is rough. A long and narrow float is preferable for rough water, as it is able to cut through the waves; but comparatively little "lift" is obtained from it. Some designers have provided their water-planes with two floats; others advocate a single float. The former makes the machine more stable when at rest on the water, but a great rawback is that the two-float machine is affected by waves more than a machine fitted with a single float; for one float may be on the crest of a wave and the other in the dip. This is not the case with the single-float water-plane, but on the other hand this type is less stable than the other when at rest. Sometimes the floats become waterlogged, and so add considerably to the weight of the machine. Thus in Mr. Hawker's flight round Britain, the pilot and his passenger had to pump about ten gallons of water out of one of the floats before the machine could rise properly. Floats are usually made with watertight compartments, and are composed of several thin layers of wood, riveted to a wooden framework.