338 . PHYSIOGRAPHY 



colder water, it is cooled, and some of its moisture is condensed, 

 producing a fog. Fogs are common along the leeward side of the 

 Gulf Stream, where the adjacent land or water is much cooler 

 than the current itself. Fogs also occur wherever warm, moist air 

 blows to cooler land. Fogs are more abundant about Newfound- 

 land than farther south, because the difference in the temperature 

 of the Gulf Stream and its surroundings is greater here than it is 

 farther south. 



Gradational effects of ocean currents. Currents have little 

 effect on the ocean bottom, and almost none on coasts, because they 

 rarely touch either. Where the water is shallow, however, as be- 

 tween Florida and Cuba, the Gulf Stream scours its bottom, some- 

 what as a great river might. Since ocean currents do little eroding, 

 they carry but little sediment. The water of warm currents 

 carries multitudes of plants and animals, many of which are very 

 small, and these organisms, or their hard parts, such as shells, are 

 scattered far and w r ide over the bottom of the ocean. 



Historical suggestions. The currents of the Atlantic played 

 an important part in the early history of America. The currents 

 southwest from the Arctic made the early discovery of North 

 America probable, after Iceland had been colonized by the North- 

 men. The south equatorial current carried the Portuguese, bound 

 for India, in 1500, to the shores of South America. 



Ocean currents were formerly of great importance in ocean 

 travel, but since steamships have taken the place of sailing-vessels 

 to a great extent, ocean currents are of less importance than for- 

 merly in directing the courses of ocean vessels. 



Tides 



Along most coasts, the ocean water rises and falls twice every 

 day, or, more exactly, every 24 hours and 52 minutes. The rise 

 and fall of the water are the tides. The tide rises for about six 

 hours, when it is high, and then falls for about six hours, when it is 

 low. The rise and fall amount to several feet in most places. In 

 bays which open broadly to the sea, but which are narrow at their 

 heads, the range is sometimes 20 or 30 feet, and in rare cases, us in 

 the Bay of Fundy, even 50 feet or more. 



