484 HYDROGRAPHICAL SURVEYING [chap. xxi. 



for scale determination, should no repetition be required, 



Y and Z at once weigh and close on X, for all ships now re- 

 quire the approximate bearing and distance of the two extreme 

 beacons (Nos. 2 and 3) before proceeding farther. 



From this Y and Z signal to X by some concise and simple 

 prearranged method (for this see also Sections VIII. and XI.) 

 theu' observations, which are repeated back to ensure the correct 

 figures having been made and received. 



Two assistants on board X, detailed as computers, at once 

 deal with such data as will suffice for the calculation of, say, 

 two independent values of the bearings and distance between 

 Beacons 2 and 3. 



This is quickly effected by the solution of the four plane 

 triangles involved, such as : X Y Z (cosine rule), Y Z 2, Y Z 3, 



Y 2 3, Fig. v., the scale for which is found by the customary 

 formula : 



Distance 1 No. of feet subtended b\^ ship x 34 



in miles J Angle subtended in seconds by ship's length 



Each simultaneous set of observations admits of sejiarate 

 solutions for the distance between tiie extreme beacons de- 

 pendent upon whether the observations of Y and Z are taken 

 separately with those of X, or combined. The accepted value 

 for this distance can be subsequently derived in the final con- 

 sideration of all the observations when time permits. 



Having tlius determined the provisional bearing and distance, 

 X communicates the same to Y and Z, when the rough plot 

 on board each ship is revised accordingly. 



SECTION V. 

 LAYING OUT BEACONS FOR TRIANGULATION. 



All three ships at once proceed, on a prearranged i)lan, to 

 drop the necessary beacons in their approximate assigned 

 positions, bearing in mind that if we are to be dependent sub- 

 sequently upon the connection of the triangulation with the 

 shore objects, then the selected position for the terminal beacon 

 cannot, in aU probability, very largely be departed from with- 



