CURRENTS. 69 



running in opposite directions. The great south equatorial 

 drift current across the Pacific Ocean, with its counter current 

 of about equal magnitude running parallel to it, may be cited as 

 an example of this. 



The currents which, in general, affect sea-works most are those 

 which owe their origin to the tides. 



The tidal wave does not produce currents unless it is inter- 

 fered with, either by reduction in the depth of the water which 

 it traverses, whereby it is changed into a wave of translation, or 

 by the configuration and irregular distribution of land, or by 

 both. 



During flood tide there is usually a slight set towards the 

 shore. It is, however, variable in direction. There is also a 

 corresponding current or drift from the shore during the ebb 

 tide. This arises from the action of the shoaling bottom upon 

 the tidal wave, whereby it is made to wheel shoreward in 

 consequence of its flank being held back by the friction of the 

 shore. The wave is thus made to approach to parallelism with 

 the coast. 



On looking at a terrestrial globe, it will be seen at a glance 

 how greatly the progress of the tidal wave is obstructed in all 

 directions, and how, in consequence, tidal currents are set up. 



Thus, to take a well-known example, the tidal compartment 

 of the North Sea is partially filled and emptied through 

 the narrow Straits of Dover. The process is, however, so slow, 

 owing to the throttling of the tidal waters, that by far the 

 greater portion of the tidal water in the North Sea has to be 

 supplied from the northern end of it. In order that this may be 

 accomplished, the tidal wave has to run northward along the 

 west coasts of Ireland and Scotland, until, arriving at the 

 Pentland Frith, it pours its waters through it, and through 

 the channels of the Orkney Islands, at a great velocity, in its 

 endeavour to equalize the respective water-levels of the Atlantic 

 Ocean and the North Sea. The current or " race " thus caused 

 through the Pentland Frith runs at times with a velocity of 

 from 10 to 12 miles per hour, that through the Straits of Dover 

 having a velocity, off the South Foreland, of about 3 miles per 

 hour. 



Numerous examples of strong tidal currents, known as " races " 

 or " roosts," may be found, especially amongst the islands on the 

 west coast of Scotland. 



