Amphibious vehicles, those that can move on both land and water, have been in use for a nu

mber of years, but while most of them were fairly fast on land, they moved quite slowly when they were functioning as boats. The only truly amphibious vehicle that can move with equal ease on both land and water is the hovercraft (气垫船). The hovercraft is the invention of an electronics engineer named Christopher Cockerell. Cockerell's hobby was sailing and he was interested in the problem of reducing the friction of water on the body of a boat, and hit on the idea of designing a boat which would travel on a cushion of air.

The air cushion under a hovercraft is produced by a large fan which blows air downwards between the craft and the water or ground, and so lifts up the craft. The air is main-rained at higher than atmospheric pressure by a flexible rubber "skirt" around the bottom edge of the hovercraft, preventing leakage of air from the cushion. Because the hovercraft floats on the air cushion with no contact between the craft and the surface below, it can travel over flat, rough ground or water with ease.

Hovercraft are usually driven by air screws like propellers (螺旋桨), which face back-wards and "push" the craft forwards, and can be turned to direct the hovercraft. Since there is no propeller dipping below the craft, hovercraft can travel up slopes out of the water, or land on beaches.

Cockerell's Air Cushion Vehicles, or ACVs, are now familiar to everyone and like all inventions, they have been improved upon. British Sea speed hovercraft have been carrying passengers and cars across the English Channel since 1968. They now have a "stretched" version of their Mountbatten Class hovercraft which can carry up to 60 cars and 416 passengers between Britain and France in a little over half an hour.

A new, large-sized hovercraft, designed and built in France, called the Sedam N500 of Naviplane, has now goneinto service. The 155 tonne N500 is 50 metres long (162 feet) and 23 metres wide (76 feet) and can carry 65 cars, plus five coaches, together with 400 passengers. When the sea conditions are ideal the N500 can reach 112 kph (70 mph).

A variation of the hovercraft principle is the sidewall ACV, which is more economical than the flexible skirt models, and easier to control, but it cannot be used on land. The United States Navy have been experimenting with warships based on the sidewall principle, and some of these may well reach a speed of 160kph (100mph).

The hovercraft ______.

A．are moved forward by propellers

B．travel on a cushion of air

C．are lifted up by a flexible rubber "skirt"

D．move faster on land than on water