373 



TRINITY HOUSE. 



TRINITY HOUSE. 



371 



degree of importance, which are under the management of the local 

 authorities, subject to the general control of the Trinity Board. 

 Harbour lights, and the buoying of the entrances to docks, rivers, and 

 navigable inland channels, are usually thug confided to the care of the 

 local authorities interested in the navigation of the particular localities 

 where those protections for shipping are placed. 



It appears from the returns of the commissioners for inquiring into 

 the subject of lighthouses, Ac., published in 1861, that, at the end of 

 there were the following numbers of lights, of the various kinds 

 enumerated, around the coasts of Great Britain and Ireland : 



The Commissioners, however, observe that the immense number of 

 bbndlj and deep bays, on the Scottish coast renders it difficult to 

 make any accurate comparison between the efficiency of its mode of 

 lighting, and that of other more regular coasts; but the table they 

 insert of the distances apart of the lights in the United Kingdom and 

 in France, has a considerable degree of interest. It seems that on the 

 English coast there is 1 light on shore for 14 miles; on the Scottish 

 coast, 1 for 39'5 miles; on the Irish coast, 1 for 34'5 miles; and on 

 the French coast, 1 for 12-3 miles. The principle adopted by the 

 French authorities in these matters is, that the lighthouses should be 



I at such distances asunder as to allow their lights t> 

 one another, and to secure this condition with lighthouses of moderate 

 elevation, it would appear to be necessary to adopt the average distance 

 asunder of the French establishments. On a plain, easy shore, without 

 rocks, or dangerous headlands, sunken reefs, or sand-banks, the distance 

 apart, thus quoted, may be exceeded without inconvenience ; but on 

 dark nights it is difficult to distinguish a light at more than 15 miles 

 ee, even when it is of the best construction, and at a great 

 elevation above the sea level. In foggy weather, or during the fall of 

 lienvy rain, sleet, or snow, the range of visibility is, of course, con- 

 siderably reduced. It is to be observed that if the English floating- 

 lights be taken into account the distance asunder of the English lights 

 does not exceed 11'37 in 



The height to be given to a lighthouse must depend upon the dis- 

 tance to which its influence is required to extend. In order to keep 

 the source of light on the level of the geometrical horizon at a distance 

 of thirty miles, Mr. Airy says that a lighthouse ought to be 5U4 

 feet high, though the Cape St. Vincent light of 221 feet high, 

 is stated in the report of the commissioners to be visible at that 

 distance. It is seldom, however, that any necessity exists for so wide 

 a range, and when light is diffused over so large an area, it can hardly 

 be brilliant in any part thereof, unless at an immense cost. Aa a 

 general rule, the horizon to be lighted is limited to about 15 or 20 

 miles, and the height of the lighthouses in such cases need not exceed 

 from 110 to 220 feet above the sea level; as will be seen by an 

 inspection of the table of lighthouses appended. The Eddystone light- 

 house, which is built upon a solitary rock in the fairway of the British 

 Channel, has a range of only nine miles, and the centre of the light is not 

 more than 90 feet above low water line. In fact, the cost of erecting 

 towers of this description in exposed situations is so great, that it is 

 important to keep their height within the limits of the strict necessities 

 of the respective localities. The average cost of a tower of about 110 

 or 120 feet high on the main land, or on rocky islands of comparatively 

 ewy access, is about from 4000/. to 10.000/., Including the illuminating 

 apparatus ; the cost of the more exposed towers is far greater, and 

 some idea may be formed of the difficult and tedious nature of the 

 works they involve from the outlay to which they give rise. Thus 

 the Bell Hock lighthouse, on the east coast of Scotland, 117 feet high, 

 cost 61,332/., nearly; the Skerry Vore, on the west coast of Scotland, 

 158 feet high, cost 83,127?. ; the Bishop's Rock, in the Scilly Isles, 145 

 feet high, cost 30.560/. ; and the Phare de Brdh.it, on the north-west 

 coast of France, cost 38,800?. ; but in this instance the expense of the 

 engineer and the superintendents, and the cost of conveyance of mate- 

 rials, were not included. M. Heynaud, the engineer of this last-named 

 tower, it may be added, dispensed with much of the complication of 

 the joints of the courses of masonry introduced by Smeaton, and pre- 

 viously followed by other engineers ; and he was thus enabled, not 

 only to effect a considerable economy in the first outlay, but also to 

 construct his tower more rapidly than his predecessors had done. 



The floating lights are, as their name implies, fixed in vessels 

 moored over or Bear to the danger it is desired to warn mariners to 

 avoid. They are usually fixed at a height of from 20 to 45 feet above 

 the sea level ; and from that reason, as well as from the difficulty of 

 introducing the best source of light (on account of the pitching and 

 rolling of the ship in heavy seas), their range of visibility is not large. 

 Practical authorities are still divided in opinion as to the best form of 

 vessels, and to the manner of mooring them ; many persons, 

 however, agree in recommending the adoption of a vessel built uppn 



the principles of Herbert's circular buoya, moored from a central chain 

 attached below the centre of gravity of the mass. Generally speaking, 

 the light-vessels on the English shores are provided with gongs, which 

 are sounded in foggy weather. The first cost of these vessels is, on the 

 average, between 4(1001. and 5000/. 



Harbour lights are of every description, according to the importance 

 of the harbour, and to the clangers of the navigable channel leading to 

 it ; sometimes they are exhibited from a small tower, sometimes from 

 ordinary gas or oil lamps. It is rarely that the most expensive of these 

 structures exceeds the cost of 4001. or 6001. ; and the range of their 

 illuminating powers is usually limited to two or three miles. 

 TABLB OF SOME op THE MOST CELEURATKD LIGHTHOUSES. 



The sources of light used are of several descriptions. In the large 

 shore lights, mechanical lamps with three concentric wicks, or pump 

 lamps, with four concentric wicks, and argand burners, are used ; in 

 floating-lights, the argand burners are alone used. The rays are con- 

 centrated and directed as may be required, either by means of dioptric 

 lenses, by catoptric reflectors, or by a mixture of the two systems in 

 the cato-dioptric apparatus; Mr. Thomas Stevenson has also intro- 

 duced an apparatus he calls the holnpholal one, which is in fact, nothing 

 but a modification of the cato-dioptric light In France, the dioptrio 

 lights are exclusively used : in England both the dioptric and the 

 catoptric lights are used for the great land- towers, whilst tho 

 catoptric lights are exclusively used for floating-lights; and the harbour 

 lights are usually made upon the holophotal system in the more 

 important ports. Mariners are divided in their opinions as to tho 

 relative merits of these systems ; but the tendency of opinion amongst 

 scientific opticians is decidedly in favour of the dioptric one, with a 

 central pump lamp of four concentric wicks. As to the distribution of 

 the light, it may be either Jijced, or by jltu/ict, or by Interval}, or 



