A place where ships may lie at anchor, secure from storms. The principal qualities of a good harbour are, sufficient depth of water to float the largest ships, and sufficient breadth and depth for them to enter with facility, and without danger of foundering. The ground should be firm, and free from rocks. It is desirable that they be surrounded by lofty hills or mountains, to screen them from high winds, and the better if so far inland as to derive therefrom increased security against being bombarded by an enemy at sea. They should also be provided with a good light-house to direct ships at night, and with numerous buoys, posts, moorings, etc. Harbours are sometimes formed artificially, either wholly or partially, by the building of moles, breakwaters, piers, and sometimes by large floating masses of timber, which rise and fall with the tide. See the articles Buoy, Breakwater, and Caisson.