Having settled the matters of procedure, it was further added that “no person in any aircraft entering the United Kingdom should carry, or allow to be carried, in the aircraft, any goods, the importation of which is prohibited by the laws relating to customs; any goods chargeable upon importation into the United Kingdom with any duty or customs, except such small quantities as have been placed on board at the place of departure, as being necessary for the use during the voyage of the persons conveyed therein, any photographic apparatus, carrier or homing pigeons, explosives or firearms, or any mails.” On the return journey the aircraft is not permitted to leave unless there be at least one British representative, approved by the authorized officer, on board. No photographic or wireless apparatus, etc., shall be carried, and no mails. Foreign, naval, or military aircraft must not pass over, nor land within any port of the United Kingdom, nor the territorial waters thereof, except on the express invitation, or with the express permission, previously obtained, of His Majesty’s Government. None of the foregoing orders applies to naval or military aircraft, belonging to, or employed in the service of His Majesty. CHAPTER XXV AERIAL COMBAT With every combat in mid-air some new theory is set up, some new conclusion arrived at, and as yet nothing can be definite. We may say for practical purposes that the strategical work is confined to seaplane and airship-scouting with the fleets at sea, and long-distance aeroplane raids into the enemy’s country; tactical work to reconnaissance trips over the neighborhood of the lines and the direction of artillery fire. The battle formation of the aeroplane squadron is now, and will in the future be similar to that of a fleet at sea. Even now the two methods of battle are closely akin. There are three distinct phases of aerial combat to be considered--aeroplane _versus_ aeroplane, airship against airship, and aeroplane against airship. It is a difficult matter to decide which is the more useful as a fighting unit, but thus far one is inclined to say the light, high-powered aeroplane. Zeppelins and airships are for the most part clumsy and unwieldly. Seaplanes, again, are usually heavy and slow to answer to their controls. The important factors are the lifting power of the machine and weather conditions. The property of “lift” is determined on the one hand by mechanical devices, and on the other by the balloon portion of the craft which is lighter than the air. Lift spells speed, endurance, and climbing powers, and therefore the machine with the greater lift is the better equipped for fighting purposes. WIND AND CLOUD