285
Order 1,000 mm, the fourth Order 500 mm, the fifth Order 375 mm and the sixth Order 300 mm.
10
However, the brightness of a light does not depend only on focal distance. Different kinds of burners and fuel also affect its power. At first coal and wood were used as fuel for lighthouses. Starting from the 19th century sperm oil derived from the whale was used but was later completely replaced by colza oil, which was cheaper. Mineral oils were used starting in the mid 19th century. The next kind of fuel was gas. Oil gas was introduced in the 1870s. Because it required huge containers for storage it was quickly replaced by acetylene and dissolved acetylene. This burns to a bright incandescence far beyond that of many other fuels. Another kind of fuel is liquid petroleum (incandescent mineral oil), which was first used at the end of the 19th century. Liquid petroleum was injected into a vaporizer, where the liquid was heated and vaporized. The vapour moves into the burner where it combusts. Normal oil burners are improved about sixfold with this burner. The latest development was electric arc lamps. Once electricity became a common commodity many lighthouses were converted to its use often using gas as a backup fuel. In Hong Kong, coal and paraffin were used in most of the lighthouses at the beginning. Only after 1915 was acetylene gas installed in Hong Kong's lighthouses.
The lights of lighthouses also have colour, usually red or green in addition to white. White is generally preferred because white lights can be made brighter. Colours are used in some sectors. White lights are used to indicate the most desirable route to port while coloured sectors indicate dangerous waters.
The distance a lighthouse light is visible depends on its height and the elevation of an observer when light intensity and weather conditions are considered equal. The lighthouse at Cape D'Aguilar has the standard height of 200 feet above mean sea level; therefore its light can be seen 23 nautical miles away by an observer at an elevation of 30 feet.
The lighthouse at Cape D'Aguilar, situated at the southeast of Hong Kong Island, was also known as Hok Tsui Lighthouse. Its tower was of stone, of conical shape, with a circular and smooth outer surface. This specific structure is typical of lighthouses in order to keep the centre...