TIDES AND OCEANIC HIGHWAYS. 



359 



of the tide it receives an accession of near forty feet to its depth of water, which enables 

 the largest West Indiamen and steamers to communicate directly with the city. The 

 change brings not only a supply of water adequate for navigation, but an alternate cur- 

 rent every twelve hours, which is just as useful as having a fair wind up and down the 

 river, the regular occurrence of which being certain may immediately be turned to 

 account by previous preparation. The same phenomenon is exhibited on the Solway 

 Firth, the sands of which are so dry at low water that travellers on horseback can cross 

 them, while the tide returns so rapidly as to render this a somewhat hazardous 



Solway Sands. 



experiment. Besides the advantage to navigation, to a 

 great metropolis like London, the tidal flux and reflux 

 of the Thames is of immense consequence to physical 

 health. The river may be said to be thoroughly washed 

 out twice a day, by which the drainage of a million and 

 a half of people is carried off to the sea, and fresh water and 

 air returned back a process which has been aptly compared to 

 a system of lungs, furnished to the city for the purpose of se- 

 curing a healthy vital action. 



A third movement to which the ocean is subject is known 

 the name of currents, which involve not merely the surface 

 stratum of the sea, but probably extend to the bottom, where they 

 prevail, and constitute great oceanic highways. The effect of currents was perceived 

 long before any thing was known of their direction and velocity ; and Columbus was 

 strengthened in his belief, that land might be reached across the Atlantic westward, by 

 substances which had been drifted from that quarter. A pilot in the service of the 

 King of Portugal, Martin Vicenti, assured him, that after sailing four hundred and 

 fifty leagues to the west of Cape St. Vincent he had taken a piece of carved wood from 

 the sea, evidently not laboured with an iron instrument, which must have floated from 

 some unknown land in a westerly direction. Columbus was also informed by his 

 brother-in-law, Pedro Correo, that he had seen a similar piece of wood off Porto Santo, 

 a small island to the north-east of Madeira, which seemed to have come from the 

 same region ; and it was commonly reported that reeds of an immense size had floated 

 to those islands from the west, which the great discoverer fancied were identical with 



A A 4 



