470 HYDROGRAPHICAL SURVEYING [chap. xx. 



portant to note that, once the initial meridian has been left, 

 the angles /3, 7, etc., formed by the sides with lines jDarallel to 

 the initial meridian, are no longer the true bearings of these 

 lines, and must never be used as such. They are usually called 

 " false bearings," and differ from the true bearings by an 

 amount equal to the " convergence of the meridians " (see p. 97 

 et seq.). It is clear that, if a chain of triangles run in an oblique 

 direction, it may be convenient to plot along and at right 

 angles to a line of known bearing, running in the same general 

 direction. 



RECENT DEVELOPMENTS. 



The efforts of scientific men to meet needs arising out of the 

 Great War have resulted in discoveries and inventions that 

 may be expected in the near futui'e to exercise a profound 

 influence on nautical surveying. 



Amongst these may be mentioned du^ectional wireless, 

 hydrophones, and the wii^e used as a ground log for measuring- 

 distances of considerable length. It is also to be anticipated 

 that photography from aeroplanes may prove to be practicable 

 for various purposes. INIodern surveying vessels will no doubt 

 be fitted with these apphances, but until further experience 

 has been obtained it is yet too early to give an account of 

 their practical use in the field. Theii" value, however, when 

 placed at the disposal of the surveyor is obviovis. 



For work out of sight of land they will be of the utmost 

 utility, but the main princij^les on which hydrographical survey- 

 ing is conducted remain unaltered, and it is in the applications 

 of the new methods to weU-estabhshed principles that develop- 

 ment may be expected. 



The description that follows of the methods for surveying 

 banks far from land, as used by Rear-Admiral F. C. Learmonth, 

 C.B., need but little modification to conform with the procedure 

 that must be adopted when hydrophones and the wire ground 

 log are at hand. 



The supreme value of du'ectional wireless and hydrophones 

 Lies in the ability they give to the surveyor of obtaining a bear- 

 ing between objects invisible from each other, within limits of 

 accuracy which are negligible at long distances. As a means of 



