444 



Proceedings of the RoyaJ Irish Academy. 



wliicli the cTiiftage tends ; but tlie lean of the trees and the driftage of 

 the beach are often in opposite diix-etions. Yre also jind that a float- 

 ing body, such as a ship at anchor, ahvays swings with the tide, except 

 in a very excessive gale ; and fishermen's nets, when they break loose, 

 always di'ift with the tidal cm-rent ; so also floating timber drifts with 

 the tide, unless it comes so near the shore as to be under the influence 

 of the wind-waves, after they have changed into " waves of transla- 

 tion." 



The study of the tidal currents on the coast of Ireland teaches us 

 that they have little or no driftage power when the tide is on the ebb, 

 even when confined in narrow channels ; to this, however, there are 

 exceptions, as the quantity of water flowing out thi'ough. a channel 

 may be considerably increased by land drainage, thus causing the 

 efflux to be of longer duration than the influx ; in some places the tide 

 runs out of an estuary for hours longer than it flows into it, the efflux 

 being augmented by floods in rivers, and the like. It appears also that 

 the chiftage is greater during spring than neap tides ; and that the 

 maximum driftage occurs when the direction of the incoming current 

 is the same as that of the prevailing wind, ^e also learn that the 

 " set" of the tidal ciu'rent in-shore depends very much, on the shape of 

 the coast line. If the coast line is straight, the direction of the set of 

 the tide along the shore, and outside in the deep water, will probably 

 be similar ; but if the coast Kne is indented, or islands lie off the coast, 

 in-shore currents back, or ''counter-tides," and cross currents, will be 

 induced, which, form off-shore banks, and thus lead to various com- 

 plications : large rivers may also form counter-currents and off-shore 

 banks. 



In a bay, fig. 1, let the normal incoming tidal wave run in the 



dii-ection of the arrow A ; tliis forms a primary cun-ent from the head- 

 land (a) to the headland (5), but from it in-shore or secondary cui'rents 

 branch off into the bay, in the directions of the arrows (c\ <?, (?'), which 

 seem to decrease in power from &■ to tr*; as the flow of the primary cur- 

 rent is usually much more rapid than that of the secondary, the tide 

 outside generally comes in faster than in the bay, consequently we often. 



* In all the woodcuts arrow A represents tlie direction 'of tlie flow tide, 

 and C the ebb tide — wbUe the aiTOWs B and D mark the directions of effective 

 winds. 



