1888.] 



On Tidal Currents in the Ocean. 



345 



should, if possible, be moored with two lines to prevent swaying. In 

 cable ships large mushroom anchors with heavy chain bridles are in 

 common use. If one of them is dropped, a ship's boat may be 

 anchored to it with no more line than is required to reach the bottom, 

 and be in no danger of swaying. Similar observations should be 

 made also from a boat anchored in very deep water. 



No measurements were made of the under current, but by sinking 

 a tow-net made fast to a sounding line, it was seen to be running in 

 the same direction as the surface current, and apparently with much 

 the same velocity. In the channels between the Canary Islands 

 where even on the shallowest ridges the water is over 1000 fathoms 

 in depth, the tidal current reaches to the very bottom, and its scour- 

 ing action is shown by the nature of the bottom. To seaward in 

 1800 to 2000 fathoms the bottom is a fine Globigerina ooze, which 

 gets coarser and sandier as the water shoals- in the channels, till on 

 the summit ridge there is generally no deposit at all, and the bottom 

 is rock or coral coated with black oxide of manganese. Round the 

 western end of Tenerife the tide runs violently causing rips and over- 

 falls. Much rocky ground is met with in the North Atlantic in 

 depths of even 1300 and 1400 fathoms, especially on the ridge which 

 extends through the whole length of that ocean. It is not unlikely 

 that the summit edge of this ridge is swept clean through the greater 

 part of its length, and it must be remembered that the removal of 

 sediment from one part of the ocean bottom means its deposit in 

 greater abundance in others, and especially in hollows in the neigh- 

 bourhood of the ridge. Hence a sounding in " ooze or clay" in one 

 position furnishes no argument against the trustworthiness of another 

 sounding in the vicinity and in equally deep water on " rock " or 

 "hard ground." Such ridges are great enemies to telegraph cables, 

 for while the tidal currents keep the rock-surface clean, they also 

 tend to give the cable an oscillating or surging motion, which is apt 

 to bring it in rubbing contact with the rock-surface and so to wear it 

 through. On the other hand these currents, in sweeping clean the 

 rocky eminences at the bottom of the ocean, prepare a lodging place 

 for deep-sea corals, and bring food to them when settled, thus enabling 

 them to build up their pillar-like banks, a very fine example of which 

 was discovered and surveyed by the " Dacia " on the 12th October, 

 1883. It lies in lat. 34° 57' N., long. 13° 57' W., and the shoalesfc 

 sounding, was 435 fathoms. The surface of the bank was locally very 

 rough, and sloped gradually to the edge in about 550 fathoms, when 

 it terminated in an actual precipice, dropping to 835 fathoms in one 

 place. 



The coral on this bank was living and growing in the greatest 

 luxuriance, and many specimens which were brought up showed that 

 the living corals were growing on a mass of dead ones. There can 



