^> 



^ ■* 



M'. 1 





t^n- 



'U 



:. I • , 



*0 wl t 



■'it 



I 



yi 



■ ;> 



J? 



'"^^^'•^^ka^r 



^ I t 



X 



w 



r 



ii 



\ f\r^ <-.. 



c ■ 



Sect. III.] 



HYDROGRAPHY. 



73 



horizon only. But if to these be added a transit and a 

 good achromatic telescope, the longitude by occulta tiers, 

 raoon-culminating stars, and eclipses of Jupiter's satel- 

 lites, will form a valuabb addition to that by observa- 

 tions of lunar distances with the sextant. 



The earliest opportunity should be taken of determin- 

 ing the error of the ehronoraetere upon mean time at the 



and evening sights, or by equal alti- 



momm 



tudes, which is better. Chronomete 



,^*Q 



will 



;oii 





change their rates on the transition from a passage in 

 which they have been constantly in motion to a state of 

 rest. Besides which, early sights afford a longer interval 

 for ratiner the watches again. 



f 



1-*- - , i ^U ^t*i i-U 



^.. 1 



' i>eni 



m^ 



;fhor- 



f ii^ ^ 



\ Li- 



_^ 



I " 



I 



iJ 



1 



,-■''■ 



i 



. J 1^ 



^J 



V 



1., 



^ V 



. 1 



:ii^ 



/^ ■ 



1^' 



J 



\i. 



n 



. ,::d^" 



-/ 



Survey of a Port 



17. A survey of the port and description of the anchor- 

 age will always be desirable if carefully made. If former 

 surveys have been executed, it will afford a useful com- 

 parison, and detect alterations of the banks and channels, 

 and the silting up of the port if any. If they have not, 

 such a survey will be doubly useful, and the industrious 

 observer will find very few plans of ports to which he 

 may not usefully add a few soundings or explanatory 

 remarks. 



It is not intended in this manual to enter much into 

 the manner of executing a survey, as there are several 

 treatises on the subject, which contain the necessary 

 information ; but these works may possibly not be on 

 board, and as " golden opportunities '' of acquiring a 

 knowledge of distant ports may thus be lost, from the 

 want of knowing how to construct a rough survey of a 



E 



