Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/649

 LIGHTHOUSE 627 action of gravity, regulated by a contrivance which maintains a constant head. If, however, the cistern be placed below this level, either a mechanical lamp is employed, in which the oil is forced into the burner by pumps worked by clockwork, or a moderator lamp, in which this is effected by the pressure exerted by a weighted piston descending in a cylinder forming the cistern. Coal-Gas. Coal-gas was first used as a lighthouse illuminant at Salvore, near Trieste, in 1817. For many years it has been used in the harbour lights of Great Britain when in the neighbourhood of gas-works. Mr J. K. Wigliam has designed a compound or crocus burner consisting of a group of twenty-eight vertical tubes, each carrying an ordinary double fish-tail burner, and the ignited gases issuing from all these jets unite into one large flame. Addi tional groups of twenty jets each can readily be arranged around the first, which forms a central nucleus, and in this way, depending on the state of the atmosphere, the power of the burner can be made at will 28, 48, 68, 88, or 108 jets. Fig. 65 shows the arrangement Fig. 65. Fig. 66. Fig. 67. for 28 jets, and fig. 66 one for 108 jets. In his triform or quadri- form systems Mr Wigham places two, three, or four of the burners already described vertically one above the other (fig. 67), with lenses opposite to each. The following table gives the candle powers, &c. , of Mr Wigham s burners. Number of Jets. Consumption of Cannel Gas per Hour. Candle Power in Sperm Candles, consuming 120 grs. per Hour. 28 48 C8 88 108 51-4 93-2 14C-3 244-0 308-0 420-6 832-0 1250-18 2408-0 29230 The diameter of the 108-jet burner is 12 inches. Faraday and Holmcs s Magneto-Electric Light. In 1853 Professor Holmes made the first magneto-electric machine for lighthouses, which was tried by the Trinity House in 1857, and the electric light was first shown to the mariner in 1858. The Trinity House sub sequently introduced it at Dungeness in 1862 and at Souter Point in 1871. The optical apparatus for these lights was designed by Mr J. T. Chance. In 1869 Holmes constructed for the Trinity House a dynamo-electric machine (in which no permanent magnet is used), giving a light of about 2800 candles. The magneto-electric light of Holmes as exhibited from a third order dioptric apparatus at the South Foreland light has been estimated at 152,000 candles, or twenty times that of the old first order dioptric fixed white light. The following table shows the results which have been obtained by Dr Tyndall and Mr Douglass by the magneto-electric and dynamo- electric machines, tried at the South Foreland, 1876-77 : i a - Light pro Orrlnr Xamc of Machines. Cost. tiH &quot;3 &quot;3 duced per I (ier of HP. in Stand 1 2SS ard Candles. Merit.

Max. Mean. Holmes s Magneto-Electric 550 3-2 400 476 476 g Alliance do. 494 3-6 400 543 S4S Gramme Dynamo-Electric (Xo. 1).. 320 5-3 420 1,257 758 4 Do. do. (Xo. 2).. 320 5-74 420 1,257 758 4 Siemens s do. (large)... 265 9-8 480 1,512 911 3 Do. do. (small, Xo. 58) 100 3-5 850 1,582 954 2 Do. do. (small. Xo. 68) 100 3-3 850 2.080 1,254 1 Two Holmes s Magneto-Electric .... 1,100 6-5 400 432 432 Two Gramme Dynamo 640 ; 10-5 420 1,085 654 Two Siemens s &quot; do. (small, Xos. E8 and 68) j-200 6-6 650 2,141 1,291 It must be kept in view that in the electric as well as in every other light the following requirements must be fulfilled: (1) it shall be constantly in sight during those periods of time at which it is advertised to the mariner as being visible ; (2) it shall be seen as far as possible in a thick and hazy atmosphere ; (3) it shall con stantly maintain the distinctive character of the station where it is employed so as not to be mistaken for another light ; and (4), when revolving, its flashes shall remain long enough in view to let the sailor take the compass bearing of the light. The electric light practically fulfils all these conditions, and, when we consider its transcendent intensity, and the smallness of the luminous radiant. which enables the engineer to adapt it to any required conditions far more strictly than oil light, we may certainly conclude that the electric is the best though the most costly of all illuminants. The only question which has not yet been decided is whether an electric light of equal initial power will penetrate a hazy atmosphere as far as an oil light. Experiments made at Edinburgh in 1866 seemed to show that the highly distinctive flash of the electric light when acted on by optical apparatus is perhaps not so much due to a greater amount of light as to the more complete parallelism of the rays arising from the smallness of the radiant. The apparatus of a small size which was first used both in England and France necessarily produced a wasteful vertical divergence, and has there fore been now justly discarded. Allard s Statistics of Lighthouse Apparatus. The following useful formulae are taken from M. Allard s very Allard s valuable Memoir, tur I IntensiU ct la Portee des Pkares, Paris, 1876. statistics. Consumption of Oil in Relation to Diameter of Burner. If c denote the consumption of mineral oil in grammes per hour, d the diameter of the burner in centimetres, then Luminous Intensities. A Carcel burner consuming 40 grammes of colza oil per hour being taken as unity, if I denote the intensity for mineral oil in a burner of diameter d in centimetres, then Luminous Intensities of Apparatus. Loss due to Reflexion, Ab sorption, and Framing of Apparatus. The loss due to surface reflexion on entering and leaving the glass may be valued at 050, 052, 058, 075, 120, 230, for angles of incidence respectively of 0, 15, 30, 45, 60, 75. In totally reflecting prisms the lumin ous ray suffers three deviations instead of two, therefore the above values should be multiplied by f. 1 The loss by absorption in the glass, although properly given by an exponential formula, may, with sufficient accuracy, be taken as &quot;03 per centimetre of glass traversed. The loss due to the horizontal joints of the lenses, and to the intervals between the reflecting rings, varies from 02 to 03, or from 01 to 04, in passing from the first to the fifth order. Coefficients. These are the ratios in which the. intensity of the lamp is increased by the apparatus. &quot;Where in is the coefficient, /the focal distance, d the diameter, and h the height of flame in a fixed light apparatus, they can, if expressed in centimetres, be calculated from the formula / f X 1 - 15 / f wt-|(-4=), or m = 2-12| J- Wd/ * Annular Lens. The intensity of an annular lens is obtained by multiplying that of the corresponding fixed light by j-^- where &amp;lt;f&amp;gt; is a the angle subtended by the annular lens, and o the horizontal semi divergence. The intensity of light from an ordinary fixed light apparatus is increased 38 per cent, bv the use of the dioptric spheri cal mirror. Distinctions of Lights. The most important characteristic distinctions of lights are the Distino- following (1) Te fixed light. Two of these are sometimes shown tions of at different heights from the same tower. ( 2) The revolving light, lights. which at equal and comparatively long periods comes slowly and gradually into full power, and then as gradually disappears. (3) The flashing light, which at short periods (such as a few seconds) comes very quickly, though gradually, into full power, and as quickly and gradually fados away, was first introduced by the late Mr Robert Stevenson in 1825. (4) The coloured light, which -is obtained by using coloured media, and was first employed by Mills of Bridlingtou. (5) The intermittent light, introduced by the late Mr R. Stevenson in 1830, which bursts instantaneously into full power, and after remaining as a fixed light for a certain length of time is as suddenly eclipsed and succeeded by a dark period. When the periods are very short the intermittent is now 1 This result as to total reflexion is not in accordance with the experiments of Professor Potter, which, however, were made with a very finely polished prism made for optical purposes.