MAEINE CUEEENTS. 343 



basia is at once more than doubled. This basin also comprises all the streams of 

 the southern states from West Florida to Texas, besides the Rio Grande del Norte 

 and the rivers of East Mexico and of South Yucatan as far as the Termines lagoon. 

 On the other hand the Caribbean Sea receives no contributions except from the 

 eastern slopes of the comparatively narrow isthmian region, and from the north- 

 west corner of the South American continent, whence come the Atrato, the 

 Magdalena and the Zulia. 



Marine Currents. 



In the American Mediterranean the tidal currents are profoundly modified by 

 the insular barriers developed round a great part of its periphery. As in the 

 Mediterranean of the eastern hemisphere, the difference between high and low 

 water is very slight, the highest tides in the Gulf near Apalachicola, in Florida, 

 averaging rather less than 4 feet, and at the harbour of E,oatan Island, in the 

 Caribbean Sea, a little over 5 feet. But the phenomenon presents great irregu- 

 larities according to the shifting character of the marine and atmospheric currents. 

 In some places the two serai -diurnal tidal waves are merged in one, and such 

 discrepancies often occur in bays or inlets lying close together. Thus on the west 

 coast of Florida the flow lasts six hours, and twelve in Apalachicola Bay on the 

 opposite side of the peninsula, while it resumes its normal period on the Texan 

 seaboard. 



Both the Gulf and the Caribbean Sea are sufficiently open to admit the regular 

 flow of the great oceanic streams; but numerous counter- currents and eddies are 

 caused by the irregular coast- lines. The vast volume of the equatorial stream, 

 which sets steadily westwards at a mean velocity of from 2^ to 3 miles an 

 hour, and which impinges on the coasts of Brazil, Guiana and the West Indies, 

 is not entirely deflected northwards, for a considerable portion is still able to 

 continue its westerly course between the islands. The current penetrating into 

 the Gulf of Paria, between Trinidad and Venezuela, is strong enough to neutralise 

 the ebb and give the flow a velocity of nearly 6 miles an hour. Thus are pro- 

 duced formidable bores, while the conflicting currents churn up the sands and 

 mud of the bay, giving the water a ruddy tinge for vast spaces. The name of 

 Boca del Drago, " Dragon's Mouth," given by Columbus to the strait between 

 the north-west extremity of Trinidad and the Paria peninsula, is confirmed by all 

 mariners navigating that dangerous passage. North of Trinidad the equatorial 

 stream flows through the strait of Tobago at a less rapid rate, averaging H 

 mile an hour, but sometimes attaining double or treble that speed. Farther north 

 access is given to the great ocean stream through other passages, and especially 

 through the channel, over 500 fathoms deep, between St. Lucia and Martinique, 



These various branches of the equatorial current, converging in the Caribbean 

 Sea, lose in velocity what they gain in expansion. Their united waters broaden 

 out to such an extent northwards that a portion returns to the Atlantic through 

 Mona Passage between Puerto Rico and St. Domingo. The normal westerly 

 movement through the Caribbean Sea is estimated at from 10 to 20 cubic miles 



