92 



HYBROGRAPHY. 



[Sect. Ill 



g^rt 



III 



hvdrography as to pliysical geograpby 



In all 



cases 



then where rivers approach or flow into any of the ports 



examination, you should acquire as extensive a 



under 



knowledge of them as you possibly can, map as much of 

 the windings and feature as is practicable, and especially 

 of such parts of those that are not navigable as may be 

 made available to the improvement of the navigation of 

 the port, or in any way be converted to hydrographic use 

 particularly noting the depth, extent, and variations of 



surface, of 



all widenings of the stream, or basins ajQTordinw 



back water and capable of being retained 



to a scouring power, carefully determinuig the elevation 

 of the surface above the meaji level of the ocean, and, if the 

 river does not run into the port, whether it could not 

 be conveyed to it, and with what facihty. These inland 

 basLis are occasionally greatly affected by mountain tor- 

 rents, melting of snow, and rainy periods, raising their 

 surface to an extraordinary height even in a few days ; 

 while, on the other hand, long dry seasons depress them as 

 much below the mean level. Our endeavour should be 



measu 



to ascertain these variations and the mean level of the 

 water of the basin ; we may often see, for weeks after 

 the event, the mark of the wash of the water around the 

 lake or basin far above the existing level ; this may be 



red and compared with the place of the mean 

 level, and be coupled with the place of the water accord- 

 ing to the best information to be procured at the place 

 (noting the informant). 



Note the depth and capability of transport or of inland 

 navigation, and the power of traversing the stream for 

 military purposes | also the nature and pecuharity of 



f 



r 



I 



CO 



nstrt 



re ( 



hav 

 tance ' 



sels, "Q 

 In 



facilit; 



bar, ii 

 rocks, 



rate a 

 and<>t 



runsn 

 water 



the tid 

 vation 



ocean. 

 Las 



the ri(^ 

 ward 



I 



stream 

 It is bi 



which 

 if a St 



necessi 

 «very i 



33. 



^unnh: 





