A river, or a sheet of water, is a conspicuous guide, and must figure plainly upon the airman’s map. Main roads, too, are clearly seen--stretching like white ribbons across the country. Railways also, and woods; while churches, chimneys, and isolated buildings may be seen from great distances when a voyager is in the air. So the airman’s map is made up of landmarks that are likely to catch his eye, remembering always his point of view from above--and remembering also that as he rushes through the air at a speed faster than an express train, he needs to identify quickly any feature of the landscape below. A sketch showing what an airman’s map is like, will be seen in Fig. 67. Here we have railways, roads, rivers, lakes, and woods all made to show plainly, and with a large building or chimney indicated here and there. With such maps, and aided by a compass, a pilot will fly for hours without losing his way. When long flights are undertaken, and it would be inconvenient for an airman to change from one map section to another, he uses a narrow strip-map, mounted upon a couple of rollers, and fitted in a case with a transparent cover. As he passes across country he unrolls his map, section by section, so that the district he is traversing lies always before his eye. CHAPTER XIV SEA-PLANES AND AIRSHIPS Biplanes and monoplanes with floats--A flying boat--The airship--Its growth from the balloon and development into types--Large craft with rigid hulls. As military flying produces special forms of aircraft, so the needs of the Navy make themselves felt; and the first task set the designers of aeroplanes was to provide a machine which should alight upon water. They did so by fitting floats, or pontoons, below an ordinary land aeroplane, these taking the place of the wheeled chassis; and then by degrees a special type of air and water craft was developed, and came to be known as the flying boat. In Fig. 39, it may be remembered, was illustrated a biplane which would rest on wooden pontoons, and so ride upon the water; and this method of a hollow float was adopted and improved by the modern builders. In Fig. 68 will be seen a typical hydro-biplane, water-plane, or sea-plane--the name last mentioned being that adopted by our Admiralty when referring to such craft. The machine is an Avro, and its appearance is that of a land aeroplane, save that it has a set of three pontoons to support it on the water--two main floats beneath the sustaining planes, and a third to bear the weight of the tail. Several needs have to be considered when such floats are built. One is that they should be buoyant enough to bear upon the water the weight of the machine, and its pilot, passenger, and fuel. In the case of the craft shown, this represents a total of 2200 lbs. Another point is that the float should detach itself readily from the surface of the water. [Illustration: FIG. 68.--An Avro Sea-Plane.