Though neither roads nor rails have to be laid and aircraft possess the great advantages of mobility and point to point transit, the initiation and maintenance of an air service is a very complex and costly matter. The utilization of converted war machines is no longer sufficient and those specially designed for commercial work are beginning to make their appearance. Such are the Handley Page W.8, the Vickers, the D.H.18 and 34, and the Bristol 10-seater. The first two are twin-engine and the last three single-engine machines. Opinions differ as to the relative advantages of the twin and single-engine type. The first and running costs of the single engine are lower, but the twin has greater power and carrying capacity, while most pilots prefer to have a surplus of power over and above that required for normal flight. For these reasons, and because of the psychological effect on insurance companies and on passengers, the twin engine will probably remain in use for large commercial machines, until long-lived and economic engines of more than 500 horse-power are available. On the other hand, where extra power is not required, the twin-engine is not safer than the single-engine machine; no existing twin-engine commercial aeroplane can maintain its height and land safely with only one engine running. Experiments have been made, especially in Germany, on the multi-engined machine with all the engines in the fuselage, but its advantages have so far been counterbalanced by loss of efficiency due to transmission gearing and shaft drives to the propellers and the vibration and weight of the gearing. High-powered engines are very expensive to run and every effort has therefore to be made by aerodynamic efficiency to carry more useful load with less horse-power. Improvement is being made in this direction; thus the D.H.18 carries eight passengers at 56 horse-power per passenger, the D.H.32 is designed for the same number at 45 horse-power each, and the D.H.34 for ten passengers at 45 horse-power each. The two best German commercial machines, the Junkers and the Fokker, have a comparatively low horse-power and a low fuel load, but greater attention has been paid to the design of the machines, which are monoplanes with cantilever wings, offering less resistance to the air than our biplanes. One of the most difficult problems is to evolve a high-lift wing which does not impair the aircraft's speed in the air. For commercial machines we must aim at the largest possible commercial load, the smallest possible fuel load and, consequently, an engine which uses fuel economically and, conversely, a lighter fuel. The development of the engine is receiving constant attention, as are also various safety devices, among which may be mentioned those guarding against fire and those varying the lift of wings so as to lower the landing speed and thus decrease the dangers attendant upon forced landings.