138 EAMBLES OF A NATURALIST. 



equal to that of 133 combined jets of gas, whilst 

 the light transmitted by M. Fran9ois' cupola is equal 

 to that of 214 jets, which would give an augmen- 

 tation of 81 jets of gas for the available light. 



MM. Arago and Fresnel commenced their obser- 

 vations in 1819 ; four years later the lamp with con- 

 centric wicks was invented, the polyzonal lenses 

 were tested, and the new mode of lighting tried in 

 the Corduan lighthouse at the mouth of the 

 Gironde, where, a century before, the first trial was 

 made of illuminating by revolving lights and by 

 parabolic mirrors. The result fully realised all the 

 hopes that had been entertained of this method, and 

 in 1825, in consequence of a remarkable report 

 drawn up by Vice- Admiral de Rossel, one general 

 plan was adopted for the lighting of the coasts of 

 France. Twenty-seven lighthouses of the first 

 order were distributed along a coast-line of about 

 1200 miles. These edifices stand forth like so many 

 advanced guards to signalise to the sailor, returning 

 from the open sea, the name of the shore he is 

 approaching. With this view these buildings have 

 been so arranged that a tower with a fixed light 

 invariably intervenes between two towers carrying 

 revolving lights, diff'ering most distinctly from one 

 another. Five lighthouses of the second order and 

 seventeen of the third, together with thirty-five port- 

 lights intervene between every two first-class light- 

 houses, to point out the dangers which are always mul- 

 tiplied in i)roportion to the increasing vicinity of land, 

 and to indicate the safe channels. Everywhere reflect- 

 ing mirrors are being replaced by this system of lenses. 



