418 HYDROGRAPHICAL SURVEYING [chap. xix. 



The shore Hne can either be sketched by the boats as they 

 go up, or done afterwards more correctly when the marks are 

 all up and fixed. Soundings, in the same Avay, can either be 

 taken from the boats as they move from station to station, in 

 which case they would cross over each time so as to get a 

 diagonal line across the channel, or can be more regularly 

 taken afterwards, as the circumstances of the case may require. 



Everything must be plotted afterwards, and communication 

 between the boats as they pass one another, when names can 

 be given and objects pointed out for mutual observation, will 

 greatly facilitate the comprehension of one another's angles, 

 when putting down the points. 



SWINGING SHIP. 



Though the compass is but little employed in surveying, it 

 is occasionally unavoidably brought into use. 



As deviation varies with lapse of time and change of latitude, 

 it must be constantly ascertained by swinging ship. 



The methods in use in swinging ship are well known, but 

 perhaps a repetition may not be thrown away. 



They are two in number : 



One, by observing the compass-bearing of a distant object 

 whose true magnetic bearing is known. 



The other, by reciprocal bearings of the compass on board, 

 and another on shore. 

 By distant The first is the best and simplest when the magnetic bearing 

 Object. q£ ^^^g distant object can be well determined. 



The object should be, at the least, six miles distant, and the 

 more the better. 



Its bearing can be obtained from observations on shore from 

 a spot in line Mdth the ship, or by true bearing with known 

 variation applied. 



Objects for this purpose are sometimes indicated on the 

 charts or in the saiUng directions, and the bearings given. 



The de\aation, for each position of ship's head, is then the 

 difference between this fixed magnetic bearing and the observed 

 bearing by compass. 



The ship can either be hauled round with hawsers, at anchor, 



