160 HYDROGRAPHICAL SURVEYING [chap. v. 



tance at wliich the shore stations can distinguish signals from 

 the ship. 



An observer with theodohte and hehostat and brass Cohorn 

 mortar is landed at A. B, C, D, E, and F are well-defined 

 peaks (see Fig. 42). 



From the ship at G a base by sound (or range-finder) is 



measured by firing from the ship and from the shore alternately; 



at each discharge the ship is shot up from A, and the angle at 



the ship between A and E observed simultaneously. A separate 



value for A E is obtained for each discharge, and its final 



2 W 



value determined from consideration of the formula T = . 



t + t' 



Ships' stations at H and K provide all the data necessary for 



plotting the points on the side A C or the longest convenient 



side. 



Example XX. — For the purposes of a rapid sketch survey 

 of a volcanic group of islands having well-defined summits, a 

 view may perhaps be obtained of a sufficient number of peaks, 

 from two theodolite stations visible from each other, and 

 situated on coast hills easily accessible on islands at a con- 

 siderable distance apart. From these stations several peaks 

 may be fixed with precision on two shots. 



The sliip, taking up positions in different parts of the group, 

 proceeds to fix herself on those peaks, and obtains third shots 

 to the remaining points ; the correctness of the work is thus 

 tested. Fixing on these points, others still more extended 

 may be shot up from different positions of the ship. If time 

 permits intermediate points may also be shot in, with a view 

 to laying down the details of the islands and plotting sound- 

 ings. The scale may be determined by patent log distances 

 between positions fixed on the points already plotted, or by 

 astronomical observations. By methods such as this, much 

 valuable information may be obtained by the officers of any 

 ship visiting regions of which little is known. 



Fig. 43 is an illustration of the above method. A is a 

 theodolite A on a low coast hill conveniently accessible, and 

 B is also a theodolite A on a summit near the coast of an 

 off-lying island. From these As the conspicuous peaks E, F, 



