84 



Notices of Memoirs — G. H. Kinahan- 



tlie trend of the coasts. The direction of prevailing winds is always 

 registered by the lean of the trees on a coast-line, Fig. 1 ; while the 

 course of the driftage is marked by the sand ridges on banks forming 

 the knee-shaped " invers " or mouths of the streams, Fig. 2 ; the 

 "inver" being shifted laterally in the direction in which the driftage 

 tends. But the lean of the trees and the driftage of the beach are 

 often in opposite directions. 



Fig. 1 .—Tree bent and growing in the Fig. 2.— Knee-sliaped 'invers' at mouth 



direction of the prevailing wind. of river, showing the set of the prevail- 



ing current. 



a. a. Travelling Shingle-"beac]a. 



We also find that such a floating body as a ship at anchor always 

 swings with the tide, except in a very excessive gale; and fishermen's 

 nets when they break loose always drift with the tides. So also 

 floating timber drifts with the tide unless it comes so near the shore 

 as to be under the influence of wind- waves, after they have become 

 '' waves of translation." 



The study of the tidal currents on the coasts of Ireland teaches 

 us that they have little or no driftage power when ih.Q tide is on the 

 ebb, even when confined in narrow channels ; to this, however, 

 there are exceptions, as the quantity of water flowing out through 

 a channel may be considerably increased by land drainage, thus 

 causing the efflux to be longer in duration than the influx ; and in 

 some places the tide runs out of an estuary for hours longer than 

 it flows into it, the efflux being augmented by floods of rivers and 

 the like. It appears also that the driftage is greater during spring- 

 than neap-tides ; and that the maximum driftage occurs when the 

 direction of the incoming tidal current is the same as that of the 

 prevaihng winds. We also learn that the " set" of the tidal current 

 inshore depends very much on the shape of the coast-line. If the 

 coast-line is straight, the set of the tide along the shore and out- 

 side in the deep water will probably be similar ; but if it is indented, 

 or islands lie off" the coast, back or " counter-currents " and cross- 

 tides will be induced — which form off-shore banks, and thus lead 

 to various complications. Large rivers may also form counter- 

 currents and off-shore banks. 



