4o8 POPULAR LECTURES AND ADDRESSES. 



scanning of the horizon for a one-minute flashing 

 light, is known to every one who has ever had 

 occasion to look for one in practical navigation ; 

 and the comparative ease of picking up a fixed 

 light, and keeping sight of it when it is found, in 

 difficult circumstances, is thoroughly appreciated at 

 sea by sailors. Still, if the revolving light can be 

 seen at all, whatever be the difficulty in picking it 

 up, and whatever the annoyance of losing sight of 

 it and having to pick it up again, it has fulfilled the 

 object of a lighthouse. All are agreed in the 

 maxim that " the grand requisite of all sea lights is 

 penetrative power ; " and if the fixed light cannot 

 be seen at a distance, or in weather in which the 

 revolving light is seen, the fixed light has failed, 

 and the revolving light has done its work for the 

 occasion. It depends very much on the special 

 circumstances whether the same quantity of light, 

 given out uniformly as a fixed light, or condensed 

 and given out in flashes, with comparatively long 

 intervals of darkness, as in the revolving light, is 

 better in respect to being seen. In stormy or 

 variable weather, with heavy showers of rain or 



