OCEANIC AND ATMOSPHERIC MOVEMENTS AND TEMPERATURE. 45 



miles an hour. Off Charleston the mean hourly flow is three miles ; from 

 there to New York it diminishes to 2^ miles, and off the Banks of New- 

 foundland it is but 1|- to two miles. The rate of flow and other characters 

 of the stream are thus largely due to the existence between the American 

 continents of the great Mexican Gulf. The Mediterranean Sea, on the 

 opposite side of the ocean, has no such effects, partly because it is on the 

 wrong side of the ocean for the production of them. 



The straits of Florida have a width of about 48 miles off Jupiter Inlet, 

 and a maximum depth of 2634 feet ; and 95 billion tons of water pass 

 per hour. The current reaches a zero velocity at a depth of 1800 to 2100 

 feet. North of the Bahamas the stream passes over the plateau bottom 

 to Charleston with a mean depth of 2400 feet, and width of 75 to 100 

 miles; and thence to Cape Hatteras, the depth diminishing to 1800 feet. 

 North of Cape Hatteras, the coastward wall is in a depth of about 390 

 feet; and inside of this wall, as well as to the eastward of the stream 

 and beneath it, flow the cold waters of the Labrador current. Owing to the 

 delay of the waters in the Caribbean Sea and the Mexican Gulf the heat of 

 the tropical waters is much augmented for distribution along their northeast- 

 ward course. 



In the central North Atlantic, between the eastward and westward parts 

 of the circuit, exists a region of calms in both winds and currents, with 

 great areas of floating seaweeds, which is called the Sm-gasso Sea. The sea- 

 weeds shelter a large variety of fishes and inferior living species. 



A belt in the equatorial region, just north of the equator, is the course of 

 a counter-current both in the Pacific and Atlantic. Currents are generally 

 made in the ocean by the prevailing winds ; and local and temporary cur- 

 rents, often of great geological importance, by the winds of a long-continued 

 storm. 



(2) Oceanic temperature. — The currents of the ocean are a means of dis- 

 tributing its heat or cold over the globe, and making cold or warm climates 

 for land and sea. Tropical currents carry tropical heat to the colder regions 

 of the globe; and, conversely, the cold- temperate and polar currents convey 

 cold ; but the former mostly as a superficial flow, seldom affecting depths 

 below 3000 feet, while the latter move at all depths from the top to the 

 ocean's bottom. The colder waters are the heavier ; but when flowing along 

 a coast region as a lagging current, they move up the shelving bottom to the 

 surface in spite of any warm waters in their way ; and whatever shoals they 

 encounter in their course, they spread up and over them, only a little affected 

 in temperature by the waters they displace. 



Superficial effects over the ocean. — As a consequence of the elliptical 

 movement pointed out, and illustrated in Fig. 26, the waters of the tropical 

 or warm side in the circuit strike the east borders of the continents ; and 

 those of the high latitude, or cold side, the west borders. They therefore 

 tend to widen the areas of warm water on the west side of an ocean, and 



