The safety of ships at sea does not depend only on the arrangements made in the ships themselves. Dangerous coasts and rocks may be seen in the daytime, 【B1】 they are a threat to safe navigation at night. Lighthouses are therefore erected at such places to 【B2】 sailors of the danger. Modern lighthouses are put up at all dangerous places near the courses 【B3】 by ships. On some rocky coasts several lighthouses may be built not far from each other, and 【B4】 the sailors on a ship may know which is which, they have 【B5】 arrangements of lights. One lighthouse will give a single flash at regular 【B6】 , another may give two flashes; a third may give two followed by one; and so on. 【B7】 watching the lights the navigator is able to decide exactly where he is, and he can set his 【B8】 with confidence. In the approaches to some ports two continuous lights are set up, one above 【B9】 but some distance behind; when they appear from the sea one above the other, the ship is on the right course; but if they do not, the captain knows that he is 【B10】 his course and likely to strike a rock or the bottom of the sea. 【B1】