34 HEIGHT OF THE WAVES. 



upheave and depress the whole mass, twice in the 

 twenty-four hours, by a regular oscillation. 



Waves raised by the wind in the open sea are rarely 

 more than 20 feet in height from the bottom of the 

 trough to the top of the crest. The greatest waves 

 known occur off Cape Horn, and in north-westerly 

 gales at the Cape of Good Hope, where they have been 

 estimated at from 30 to 40 feet in perpendicular 

 height, which may be taken as the maximum height 

 of an unbroken sea-wave or swell. It is true that 

 poets and travellers tell us of waves " mountains high ;" 

 but we must make allowance for a terrified imagina- 

 tion, and for the natural desire to make the perils we 

 have safely passed appear impassable to our tarry-at- 

 home friends. Besides the exaggeration due to these 

 causes, another source of error may be found in the 

 movement of the ship, as she rises over the " moun- 

 tain," or plunges into the " abyss" in her wild career. 

 Where the base varies at every instant it is scarcely 

 possible to estimate the dimensions of the masses that 

 pass under us. 



Currents, though to the common observer much 

 less obvious than waves, are developed on a scale in- 

 finitely greater, and are the main agency employed in 

 creating a healthful circulation through the depths of 

 the sea. We call them upper or under currents ac- 

 cording as they carry along the surface waters or move 

 the strata beneath. Some constantly set in one direc- 

 tion, whilst others are reversed at opposite seasons of 

 the year. Several of the upper currents, which depend 

 on particular winds, are of this class. Of the peren- 



