Page:History of merchant shipping and ancient commerce (Volume 3).djvu/341

 *ment of lighthouses, In the Exhibition of 1851 the French exhibited some beautiful specimens of coast-lights, in which they then excelled, but, since then, (see papers read by Sir William Thompson and Mr. J. Hopkinson before the British Association at Bristol) we have made remarkable improvements in the forms of our lighthouse apparatus, and now produce lights more powerful and brilliant than any other country. These are, chiefly, manufactured by Messrs. Chance Brothers of Birmingham, and, for the mode of arranging the glass reflectors, we are greatly indebted to the genius of the late Professor Faraday and Sir William Thompson. Lights are now constructed, which on a clear night can be seen at a distance of twenty-five miles, perhaps more. But still greater improvements have, since then, been made by arranging the colours, or rather the variation, of lights along a line of coast, so that the navigator may be able, at once, to distinguish one light from the other. For instance, some are fixed, single or double, white, red, or flash lights, or are revolving, displaying alternately these or other colours. But it has been found that red glass absorbs nearly two-thirds of the power of the light, and thus is to a very large extent deprived of its usefulness. Indeed, it has been found that colour of any kind used to distinguish one light from another materially lessens its power. Consequently, we are now adopting other means to distinguish one light from another on any given line of coast. That is, we make eclipses of opaque shades revolving round the usual lighting apparatus, and these we can vary so as to show 5, 10, 15, 20 or 30 seconds of darkness with similar or greater intervals of bright light. We may thus use altogether white or bright lights, which have the greatest power to work in such a manner that one can be easily distinguished from another. I may add that electric light, instead of that produced by oil or gas, has been tried within the last few years. One of these lights was fixed in 1871 on Souter Point, coast of Durham. The flashes were of 5 seconds' duration, with dark intervals of 25 seconds. The apparatus producing this effect consisted of a dioptric of the third order for fixed lights, around which there was an octagonal drum of glass, consisting of panels of eight vertical lenses; by these the divergent and continuous sheet of light from the fixed portion of the apparatus was gathered up so as to form distinct beams which successively reach the observer as the panels pass in succession before him. The electricity for the production of the spark was generated by one of Professor Holmes' magnetic-electric machines, worked by a steam-engine of four or five horse-power. buoys, and beacons, whether under the immediate control of the Ancient Trinity House of Deptford Strond, of the "Commissioners of the Northern Lighthouses," or of the Dublin "Bal-*