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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- 
SEO. SER. 12 
