262 ELECTRIC LIGHTING. 



commutator, but to centralize the two points of light is a 

 more difficult matter. 



The carbons used in lighthouses are 7 millimetres wide 

 and 27 centimetres long, and their rate of consumption is 

 estimated at 5 centimetres per hour at each pole, at least 

 with machines giving alternating currents. In spite of this 

 uniform consumption there is, however, a slight difference., 

 and the upper carbon is consumed a little faster than the 

 lower, in the ratio of 108 to 100. The regulators are there- 

 fore very well adjusted, but as it is important that the varia- 

 tion of the luminous point shall be under 8 millimetres, 

 without which the rays will not be sent out to the horizon, 

 it is necessary to carefully attend to this light In order that 

 the keepers may be able easily to observe the progress of 

 the carbons, an image of them is projected on the wall by 

 means of a lens of short focus, a horizontal line is marked 

 on the wall, and the points must be equally distant from that 

 line. As a deviation of i millimetre is represented by a 

 deviation of 22 millimetres on the wall, defects in the adjust- 

 ment are readily seen. 



This apparatus began to work at the lighthouse on the 

 south cape of La Heve on the 26th December, 1863, and 

 after fifteen months of experiments it was decided -to apply 

 the same plan of lighting to the second lighthouse. From 

 that time electric lighting was definitely established. 



As to the machines, which, like regulators, are provided 

 in duplicate, they are generally placed in the base of the 

 lighthouse tower, with the steam-engines for driving them, and 

 well-insulated cables of a large diameter convey the current 

 to the regulators, as we have already said. 



According to Le Roux, it seems that even with the Alliance 

 machines of 4 discs, the cost of the light is, on the average, 

 one-seventh of that of oil. 



In the natural state of the atmosphere the Alliance machines 

 with 4 discs give a light visible at 38 kilometres, and those 

 with 6 discs have a range of 50 kilometres ; but it is curious 



