Sect. TIL] 



HYDKOGRAPHY 



91 



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flPected. It is 



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sage can be ettectecl. It is the sea getbi 



breeze and beating over the reefs into the lagoon that 



occasions such a current 



through the 



openm 



Inquire 





into this on the spot, and do not commence any tide 

 observations in the lagoon if the reefs are low and 

 channel small : if, however, the lagoon be open on one 

 side and sheltered on the side of the prevailing wind, these 

 spots in the ocean afford excellent places for observations 



upon the tides. 



Currents occasioned by the trade-wind prevail about 

 all the islands situated in those latitudes ; their direction 

 and force should be ascertained and stated in your re- 

 marks. 



Riversi. 



32. All rivers should be traced to the furthest possible 

 point that time will allow, for although it is the usual 

 practice to limit hydrographic inquiry to the vanishing 

 noint'Of tidal influence, yet there are many reasons why 



X 



we should not here so circumscribe our views. Rivers 

 are the great arterial features of our globe ; they define 



the valleys, give boundaries to the hills and mountain 

 ranges, and if traced to their source enable us, with the 

 aid of a few well-determined culminating points of con- 



tiguous ranges, to trace upon our charts the general 



feature of the country through which they flow. Besides 

 which they arc so far connected with the navigation of 

 our ports and harbours that their aid is often indispensable 

 to a free access and egress, by affording a powerful means 



of scouring channels and removing impediments to ship- 

 ping, which would otherwise be denied admission* They 

 may therefore be said to be of almost equal importance 



