260 ELECTRIC LIGHTING. 



the ist December, 1877, gives many details of the manner 

 in which the apparatus is fitted up in the lighthouses at 

 Lizard Point. This would be very interesting to describe, 

 but for want of space we shall at present content ourselves 

 with considering the way in which the electric light is arranged 

 at the top of the lighthouses. 



The illuminating part of a lighthouse is constructed, as 

 shown in Fig. 68, of a glass lantern formed of a certain num- 

 ber of Fresnel's lentilles-a-echelons (lenses in steps), in the 

 centre of which the luminous focus is situated. This lantern 

 is turned by powerful clockwork, and it is the passage of the 

 separating zones between the different lentricular parts that 

 produces those eclipses by which the light of a lighthouse is 

 distinguished from that of an ordinary fire. The smaller the 

 luminous point the more the effect is magnified by the lenses, 

 and in order that the light shall be visible from a great dis- 

 tance it is essential that the luminous focus should be as 

 bright and as small as possible. Now, the electric light solves 

 this double problem, and therefore it appears expressly made 

 for lighthouses. Nevertheless, as the electric light regulators 

 are occasionally liable to extinction, and a prolonged ex- 

 tinction might cause serious disasters, the regulators (gene- 

 rally Serrin's or Siemens') are arranged in duplicate. They 

 slide into the lantern on small rails placed on the surface of 

 a cast-iron table, a stop arrests them when at the focus of 

 the lenses, where they instantly light themselves, and this 

 is one of the great advantages of the electric light, espe- 

 cially with the regulators to which we refer. - The electric 

 communication is established on one hand by means of the 

 cast-iron table, and on the other by a metallic spring which 

 presses against the upper part of the lamp at a convenient 

 point. The substitution of one lamp for another does not 

 require more than two seconds, the one which is withdrawn 

 pas:>mg out by one of the railways whilst its substitute 

 enters by the other. The light may also be made to pass 

 instantly from one apparatus to the other by means of a 



