CHAP. XIX.] EXPLORING A RIVER 417 



reaches of the river, will much assist us, especially when, 

 which is so often the case, the river is tliickly lined with trees 

 and jungle. 



The patent log will be fitted, as already described, with the 

 dial on the gunwale and the fan towing astern. Theodolite 

 legs standing in the stern-sheets make an excellent stand for 

 a prismatic compass, and enable us to get a better bearing 

 than by holding it in the hand. 



It is in rounding the bends that the greatest error in map- 

 ping a river is introduced, as the distance run over while 

 gradually altering course must be estimated by eye, which 

 requires considerable experience at judging chstances. 



Current must be taken into consideration, and may be Current, 

 obtained, if time allows, by anchoring the boat for half an 

 hour, and reading the patent log, or in shorter time, by heaving 

 the current log. 



In a river where the tide extends some distance up, and 

 where the land is low and jungly, as in so many mangrove 

 rivers, our difficulties are much increased, as the velocity of 

 the current \\ill be constantly varying, and we cannot hope 

 to obtain any sextant fixes to check our position. In cases of 

 this kind, if it is desired to have any degree of accuracy in the 

 sketch, the only way is to run over the work again, making an 

 independent map, and squaring in afterwards a mean of the two. 



It is best always to plot as we go. Mistakes are thus rendered Plotting 

 less likely, and the vexed question of the bends can best be ^" 

 solved by placing their shape on the paper at once. We can 

 also look at our work on the way down again, and correct little 

 inaccuracies more readily. 



If it is desired to make a large-scale plan of a river of greater Survey on 

 width, the best method is to employ several boats at once, g^^f^g^ 

 four if possible, which will triangulate their way up, two on 

 either side. Starting from two fixed points at the mouth of 

 the river, two boats will remain there while the other two go 

 up to convenient positions, whence they can see the boats 

 remaining at the first points. Angles will then be taken from 

 all, to everything conspicuous, and to one another, and the 

 lower boats will, leaving marks at their old stations, move up 

 to two new positions above the other boats, when the angles will 

 be repeated, and so on, the lower boats moving on each time, 



27 



