

49G 



CURRENTS. 



[Ch. XX. 



rent. 



is accumulat 

 sle may be ei 

 formed. A 



may 



bulk, or depth, or velocity; a drift-current is shallow and 



mile 



Currents flowing alternately in opposite directions are 

 occasioned by the rise and fall of the tides. The effect of 

 this cause, as we shall see in the sequel, is most striking in 

 estuaries and channels between islands. 



heat. 



rd cause of oceanic currents 

 Of this the current setting 



from the Atlantic 

 e Mediterranean is 



exam 



chapter. It must happen in many other parts of the world, 

 that large quantities of water raised from one tract of the 

 ocean by solar heat, are carried to some other where the 

 vapour is condensed and falls in the shape of rain, and the 

 surface waters thus increased will give rise to currents as 

 they flow back again to restore equilibrium. 



These considerations naturally lead to the enquiry whether 

 the level of those seas out of which currents flow is higher 

 than that of seas into which they flow. If not, the effect 

 must be immediately equalised by under-currents or counter- 

 currents. Arago is of opinion that, so far as observations 

 have gone, there are no exact proofs of any such difference 

 of level. It was inferred from the measurements of M. 

 Lepere, that the level of the Mediterranean, near Alexandria, 

 was lower by 26 feet 6 inches, than the Red Sea near Suez 

 at low water, and about 30 feet lower than the Eed Sea at 

 the same place at high water,* but the late Mr. Robert 

 Stephenson affirmed, as the result of a more recent survey, 



t 



imae 



greater, diversity in the relative levels of the Atlantic and 

 Pacific, on the opposite sides of the Isthmus of Panama. 

 But the levellings carried across that isthmus by Captain 

 Lloyd, in 1828, to ascertain the relative height of the Pacific 



Steam Communication with India, Juty 



* Ann. du Bureau des Long. 1836. 



t Second Parliamentary Report on 1851. 





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