1888.] 



On Tidal Currents in the Ocean. 



341 



tween the solar and lunar day. From the nature of the observations 

 the effect due to this would not be easily detected. It is evident then 

 that the ordinary method of observing oceanic currents is such as 

 completely to cloak any tidal effect which may exist. The proximate 

 source of energy for the production of tidal currents is the tidal 

 wave. 



Storm waves are confined to the surface of the ocean, and break 

 only in comparatively shallow water. The tidal wave affects the 

 deepest oceans to the bottom. It might, therefore, be reasonably 

 expected that, in passing over many of even the deeper ridges which 

 traverse the ocean bed, its character as an undulation would be 

 modified with the production of a true tidal current. We know that 

 in the shallow water surrounding the land and in the bays and inlets 

 which indent its coasts, a portion of the tidal energy is dissipated by 

 "the partial transformation of the wave into currents. 



Id littoral waters these currents are necessarily exaggerated by the 

 confinement produced by the neighbouring land; but the presence of 

 a shoal alone, without any dry land in the vicinity, ought to be 

 sufficient to produce well-marked and regular tidal currents. 



Considerations of this ' nature determined me to take the first 

 opportunity which might offer of putting the matter to the test of 

 observation. 



Thanks to the hospitable invitation of the India Rubber, Gutta 

 Percha, and Telegraph Works Company, of Silvertown, I had the 

 good fortune to spend the months of October and November, of 1883, 

 on board the s.s. "Dacia," one of their excellently equipped cable 

 ships, which, along with the s.s. " International," was sent out to 

 connect Cadiz with the principal islands of the Canary Group by 

 means of a telegraph cable. The whole expedition was under the 

 command of the Company's Telegraphic Engineer-in-chief , Mr. Robert 

 Kaye Gray, to w»hom I am particularly indebted for the facilities 

 which were afforded me in carrying out this and many other investi- 

 gations, and I beg publicly to tender him my best thanks. 



In the course of the sounding operations carried out with a view of 

 gaining a thorough acquaintance with the depth and nature of the 

 sea bottom, over which it was proposed to lay the cable, many 

 remarkable inequalities were met with. Perhaps the most striking 

 was one which was called the " Dacia Bank," after the ship on which 

 it was discovered. This bank, which occupies a surface of 50 square 

 miles with less than 100 fathoms of water on it, rises rapidly from the 

 prevailing depth of 1800 or 1900 fathoms to within 500 fathoms of the 

 surface, whence the slope is very abrupt and in many places precipitous 

 to within 100 fathoms of the surface. As the bank lay close to the 

 proposed route of the cable, two days were spent in surveying it care- 

 fully. In order to. have a fixed point to refer the soundings to, a 



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