514 



HYDROGRAPHICAL SURVEYING [chap. xxi. 



Beacon 3 

 Qj-opped bn ^/^ 



Ship Y 

 At single ^ 



Case I. 



Three Ships present 



X. Y. and Z. 

 X being the CcntroUing Vessel 

 and not less than zoo feet long. 



Ship Z 



» t single ■t' 



Fig. v.— Obtaining the Scale provisionally. 



