LIGHT-HOUSES. 265 



near the land alone that the dangers are seen to com- 

 mence. 



Such danger occurs in ports into which no prudent 

 sailor would enter without a pilot ; it occurs where, even 

 with this help, no one would risk attempting to penetrate 

 at night ; we easily see, then, how indispensable it is, if 

 we would avoid irreparable accidents, that after sunset 

 signals of flame, easily visible, should indicate on all sides 

 the proximity of land. It is necessary moreover that 

 every ship should perceive the signal far enough off for it 

 to find, in evolutions often sufficiently difficult, the means 

 of keeping itself at some distance from the shore until the 

 moment when day shall appear. It is not less desirable 

 that the different lights which we kindle along a certain 

 extent of coast should not be confounded with each other ; 

 and that at first sight of these hospitable signals the navi- 

 gator, who by an unfavourable sky has been for some 

 days deprived of the means of directing his course, should 

 know, for example, on returning from America, whether 

 he is about to enter the Gironde, the Loire, or the har- 

 bour of Brest. 



On account of the roundness of the earth, the range of 

 a light-house depends on its height. In this respect men 

 have always obtained without difficulty the range which 

 the wants of navigation demanded : it was a simple ques- 

 tion of expense ; every one knows, for instance, that the 

 great edifice with which the famous architect Sostrates 

 of Cnidus adorned the harbour of Alexandria, nearly 

 three hundred years before our era, and most of the 

 light-houses constructed by the Romans, were of consid- 

 erably greater height than the most celebrated modern 

 towers. But in an optical point of view, these light- 



SEC. SEE. 12 



