132 EAMBLES OF A NATURALIST. 



nearly the same diameter as the mirror itself. Com- 

 pared with the area of the sea, it was like one simple 

 ray of light, which could only render the light- 

 house visible to a spectator standing in a direct line 

 before it. 



Borda's invention would therefore have proved of 

 no use if it had not been for a very ingenious idea 

 suggested by a person named Lemoine, who had 

 formerly been Mayor of Calais. His plan consisted 

 in placing Borda's apparatus on a moveable axis, 

 which, by its rotatory movement, presented the 

 mirror to every part of the horizon in succession. 

 The observer, who is placed at a great distance from 

 the lighthouse perceives it only during the time 

 required for the cylinder of reflected light to pass 

 before his eyes, the building at other times seeming 

 to him to be shrouded in complete darkness. This 

 latter circumstance, far from injuring the effect pro- 

 posed, presents, on the contrary, great advantages. 

 By arranging round one and the same axis a certain 

 number of reflectors, which are all provided with 

 their respective lamps, each revolution of the ma- 

 chine will present as many luminous flashes as there 

 are mirrors, while a certain time will elapse between 

 their successive occurrences, during which the spec- 

 tator is plunged in obscurity. By varying the 

 number and duration of these intervals, one may 

 give a special character of individuality to a certain 

 number of lighthouses, which is in itself a very 

 essential condition, since it is only by this means 

 that vessels arriving from a distance are able to 

 recognise the precise point of the shore in sight, and 



