MARINE CURRENTS 231 



are very small, that of Lake Michigan, for example, having an 

 interval of only 2 inches. More irregular oscillations of the entire 

 water body of lakes arc found in the seiches, which are due to 

 some disturbance of the water as a whole, as in the case of sudden 

 barometric changes, in storms, etc., and may be compared to the 

 oscillation of the water in a basin which has been lifted on one side 

 and suddenly dropped. Such oscillations will continue for some 

 time after the cessation of the disturbing force. 



Tidal Scour and Transportation. Wherever the tide passes 

 through a narrow channel so as to produce a race, considerable 

 scouring of the bottom results. Thus the tidal stream channels in 

 salt marshes are kept open by this tidal scour, and not infrequently 

 harbors are benefited by such scour. Transportation of sand and 

 mud by tidal currents is often extensive. Sand grains i/ioo inch 

 (about 0.25 mm.) will be moved in a state of semisuspension by 

 tidal currents of 3 or 4 knots. The movement is, however, princi- 

 pally a forward, backward movement, the sands carried away by 

 ebb tide being brought back during flood. This is illustrated by 

 the case of a vessel which sank at the mouth of the Gironde, 

 opposite Verdun, where she rested on her keel at the bottom of the 

 channel in 6 fathoms of water at low tide. "At the end of the ebb 

 tide the sand was so scoured as to leave a space under the keel at 

 both ends, leaving the hull only supported in the middle ; at the 

 end of the flood tide the vessel was again completely buried in the 

 sand, the sand bed extending 100 yards fore and aft of the vessel 

 and 50 yards from each side." ( Partiot-50 and \'Vheeler-73 :/d.) 



Marine Currents.* 



Currents of the Oceans. Ocean currents are due to a com- 

 bination of causes, which may be classed either as primary or as 

 secondary stream constituents. The primary causes are the active 

 producers of the currents, and as such may be noted (i) internal 

 or endogenetic causes, existing within the water itself, such as 

 local ditlerences in density, due to variation in temperature and 

 salinity under the influence of varying sunshine, evaporation, rain- 

 fall, or melting of snow and ice; (2) external or exogenetic causes, 

 such as variation in air pressure, and especially the winds. Among 

 the secondary causes which act chiefly in modifying the current 

 may be noted, i, friction, 2, the rotation of the earth on its axis,"j" and 



* In this section I have followed Kriimmel closely. 



t This operates to deflect all moving particles on the surface of the Northern 

 Hemisphere to the right, and to the left in the Southern. 



