24 THE OCEAN. 
him from the overwhelming wave. Through the 
Pentland Frith, between Scotland and the Orkney 
Islands, the spring-tide rushes at the rate of nine 
miles an hour. The tide in inland seas is so slight 
as to be scarcely observable, probably owing to the 
smallness of the volume of water which they con- 
tain; and hence the astonishment which the soldiers 
of Alexander, accustomed to the equable condition 
of the Mediterranean, felt, when at the mouth of 
the Indus, they beheld the sea swell to the height 
of thirty feet. 
That some purpose, important in the constitution 
of our world, is effected by these periodical ebbings 
and flowings of the mighty sea, is highly probable; 
but our acquaintance with the arcana of nature is 
too slight to point it out. In navigation they are 
useful; the flood-tide permitting ships to sail up 
rivers, even when the wind is adverse, and often 
admitting deep vessels to pass into harbors, over 
banks or bars, impassable at the ordinary depth of 
the water. , 
Besides the tides, the sea has other motions of 
great regularity, called currents. The principal of 
these is the notable Gulf-stream, a strong and rapid 
river, as I may say, in the sea, whose banks are 
almost as well defined as if they were formed of 
solid earth, instead of the same fickle fluid as the 
torrent itself. It first becomes appreciable on the 
western coast of Florida, gently flowing southward 
until it reaches the Tortugas, when it bends its 
course easterly, and runs along the Florida Reef, 
increasing in force, till it rushes with amazing 
