HARBORS, &c. 109 



changes its color. There are : 1. Light-houses with stationary, intermitting 

 lights. The Trieste light-house is one of these. PL 32, fig. 13, is a front 

 view of this light-house ; fig. 14, a vertical section. The lower part of the 

 tower is a casemated fortification, for the protection of the harbor. Fig. 15 

 shows the ground plan of the basement, in the direction of the line AB ; 

 fig. 16, that of the casemates in the direction of the line AB in fig. 14. The 

 lighting apparatus is represented in^^. 17 : p,p\p^ are frames for the station- 

 ary lamps, in a lantern provided with glass windows. On the stationary post k 

 is a frame, Imno, resembling an umbrella, to which is attached a perpendicular 

 screen q, covering one half of the inside of the tower. This frame is made 

 to revolve by the wheels abed and fghi, which are moved by clock-work, 

 so that the screen, q, at one time leaves the lamps free and then again con- 

 ceals them. 2. Light-houses with revolving, intermitting lights. The Bell 

 Rock light-house, which was erected in 1811 on one of the most dangerous 

 rocks near Dundee, is of this kind. Fig. 18 is a vertical section. The tower 

 is washed by the waves, and the entrance, consequently, is above the range 

 of the breakers at B ; A, C, D, and E are the different stories of the light-house, 

 in which the keepers live. At H is the watch-room ; G is the lantern ; K is 

 a flag-stafT, on which a signal flag is raised in the day-time. The lighting 

 apparatus consists of an upright axle, which is turned by means of the clock- 

 work F, and turns with it a frame, of which one half is a semi-cylindrical 

 screen, plated on the inside, and highly polished, while the other half bears 

 seven large and brilliant Argand lamps. As the axis revolves, the dark 

 side of the screen and the burning lamps are presented alternately. 3. Light* 

 houses with revolving, intermitting, colored lights. Of this kind is the Cor- 

 douan light-house at the mouth of the Garonne {fig. 23). The polyzonal 

 lenses, I, invented by Fresnel are here used, by which nearly all the rays 

 of light are thrown in parallel lines, while those which fall above and below 

 are also thrown into parallel lines by the parabolic reflectors H and K. 

 The apparatus consists of eight lenses, in the focus of which is a large 

 Argand lamp, four inches in diameter, with its chimney, L. Four of these 

 lenses are shaded green. The whole apparatus, with its foot, D, stands on 

 the plate of the column, B, supported by the wheels, g. Above g is the 

 cog-wheel /, in which plays the pinion, e, of the clock-work, E, which is 

 moved by the axis d; be is a regulator with arms and conical pendulum. 



The necessity of erecting light-houses in distant places, where skilful work- 

 men and the requisite building materials are not to be had, has suggested to 

 the English the idea of iron light-houses, which can be taken in separate pieces 

 to their places of destination and there put together. PL 32, fig. 19, repre- 

 sents a section of an iron light-house constructed in London a few years 

 since for the Bermuda Islands. The foundation up to the first story is 

 built of stone, although the iron work commences in this portion of the 

 building. In the second story the wall is much lighter, and is plated with 

 iron plates on both sides. From the third story upwards iron plates only 

 are used, which are shown in their upright joints (fig. 21) and in their 

 horizontal ones (fig. 20). They are fastened with cast-iron flanges on the 

 inside by strong iron screws. The stairs, floors, window frames, and 



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