So far, when he tours the air, a pilot has few restrictions. He must not fly above certain areas, it is true, but in the main the sky is free to him, and as yet he need not register or number the craft he buys, like the owner of a motor vehicle has to do. The fact is, however, that laws for the air have been studied, and in some cases a schedule prepared; but no definite scheme of rules has, up to the present, been placed upon the Statute books. The intention is, indeed, to allow the science to develop with as few limitations as may be possible. There are so many problems to be solved in regard to the governing of flight, that they can only be dealt with as they arise. The air, as an element in which men may navigate a craft, has aspects which are unique. Oceans and continents, for example, all have their limitations: the land ends and the sea begins, and then the sea in its turn gives place again to land. The traveller from London, say, to Paris, first passing from inland to the sea, has to leave the train and embark upon a boat, only to change again into a train and finish his journey--as he began--by way of the land. But an aerial voyager between the two cities finds no restrictions such as these: the air has only one boundary--the point at which it touches the surface of the earth; and so he may rise from London, and fly with equal facility above land or sea. Although the air has no limitations, such as are encountered by land or sea, there has been discussion between nations as to the rules they should impose to control the flight of craft. It has been agreed, without question, that the air-space over the high seas should be regarded as free, and also above territory that is unoccupied. The idea of an aerial law which is thought most reasonable, indeed, is that the sky over the whole globe should be declared free for traffic, but that each nation should be allowed to enforce rules to prevent attack or spying from above, and to check aerial smuggling. But that flying over a frontier might be controlled by a corps of aerial police is considered hardly feasible: it would be almost impossible indeed--had he made up his mind to do so--to prevent an airman from crossing above some boundary. Even were there patrol craft on duty, he could rise high among the clouds and so escape detection.