Besides the high towers there are many minor beacons, light-ships, and light-buoys in use. Many of these are untended and therefore must operate automatically. The light-ship is used where it is impracticable or too expensive to build a lighthouse. Inasmuch as it is anchored in fairly deep water, it is safe in foggy weather to steer almost directly toward its position as indicated by the fog-signal. Light-ships are more expensive to maintain than lighthouses, but they have the advantages of smaller cost and of mobility; for sometimes it may be desired to move them. The first light-ship was established in 1732 near the mouth of the Thames, and the first in this country was anchored in Chesapeake Bay near Norfolk in 1820. The early ships had no mode of self-propulsion, but the modern ones are being provided with their own power. Oil and gas have been used as fuel for the light-sources and in 1892 the U. S. Lighthouse Board constructed a light-ship with a powerful electric light. Since that time several have been equipped with electric lights supplied by electric generators and batteries. Untended lights were not developed until about 1880, when Pintsch introduced his welded buoys filled with compressed gas and thereby provided a complete lighting-plant. With improvements in lamps and controls the untended light-buoys became a success. The lights burn for several months, and even for a year continuously; and the oil-gas used appears to be very satisfactory. Recently some experiments have been made with devices which would be actuated by sunlight in such a manner that the light would be extinguished during the day excepting a small pilot-flame. By this means a longer period of burning without attention may be obtained. Electric filament lamps supplied by batteries or by cables from the shore have been used, but the oil-gas buoy still remains in favor. Acetylene has been employed as a fuel for light-buoys. Automatic generators have been devised, but the high-pressure system is more simple. In the latter case purified acetylene is held in solution under high pressure in a reservoir containing an asbestos composition saturated with acetone. The light-sources of beacons have had the same history as those of other navigation lights. Many of these are automatic in operation, sometimes being controlled by clockwork. During the last twenty years the gas-mantle has been very generally applied to beacon-lights. In the latter part of the nineteenth century a mineral-oil lamp was devised with a permanent wick made by forming upon a thick wick a coating of carbon. The operation is such that this is not consumed and it prevents further burning of the wick.