[Illustration:  A. A COMPLETED LIGHTHOUSE LENS]

[Illustration:  B. TORRO POINT LIGHTHOUSE, PANAMA CANAL]

[Illustration:  AMERICAN SEARCH-LIGHT POSITION ON WESTERN FRONT IN 1919]

[Illustration:  AMERICAN STANDARD FIELD SEARCH-LIGHT AND POWER UNIT]

Doubtless, the reader has noted that reliability, simplicity, and low cost of operation are the primary considerations for light-sources used as aids to navigation.  This accounts for the continued use of oil and gas.  From an optical standpoint the electric arc-lamps and concentrated-filament lamps are usually superior to the earlier sources of light, but the complexity of a plant for generating electricity is usually a disadvantage in isolated places.  The larger light-ships are now using electricity generated by apparatus installed in the vessels.  There seems to be a tendency toward the use of more buoys and fewer lighthouses, but the beam-intensities of the latter are increasing.

In the hundred years since the Boston Light was built the same great changes wrought by the development of artificial light in other activities of civilization have appeared in the beacons of the mariner.  The development of these aids to navigation has been wonderful, but it must go on and on.  The surface of the earth comprises 51,886,000 square statute miles of land and 145,054,000 square miles of water.  Three fourths of the earth’s surface is water and the oceans will always be highways of world commerce.  All the dangers cannot be overcome, but human ingenuity is capable of great achievements.  Wreckage will appear along the shore-lines despite the lights, but the harvest of the shoals has been much reduced since the time described by Robert Louis Stevenson, when the coast people in the Orkneys looked upon wrecks as a source of gain.  He states: 

It had become proverbial with some of the inhabitants to observe that “if wrecks were to happen, they might as well be sent to the poor island of Sanday as anywhere else.”  On this and the neighboring island, the inhabitants have certainly had their share of wrecked goods.  On complaining to one of the pilots of the badness of his boat’s sails, he replied with some degree of pleasantry, “Had it been His [God’s] will that you come na here wi these lights, we might a’ had better sails to our boats and more o’ other things.”

     In the leasing of farms, a location with a greater probability
     of shipwreck on the shore brought a much higher rent.

XIV

ARTIFICIAL LIGHT IN WARFARE