50 



HYDROGRAPHICAL SURVEYING [chap. i. 



Height of 

 Masthead. 



Scaffold- 

 ings. 



Light- 

 houses. 



In preparing for a surveying cruise, therefore, provision of 

 material for marks must not be forgotten. 



This should be accurately measured perpendicularly from 

 truck to water-line, and to hammock netting or rail, by tricing 

 up the measuring chain. 



If circumstances require it, a scaffolding 80 or 90 feet high 

 may be erected on which to mount a theodolite. An outer 

 scaffolding is necessary for the observer to stand on, and an 

 inner one quite separate from the other for the theodolite. 



The heads of the poles forming the four corners of each 

 scaffolding are inclined inwards towards each other at an 

 angle of about 10 degrees from the vertical, and they must 

 be suitably braced and tied to give rigidity. It is, however, 

 but very rarely necessary to resort to such expedients in 

 Hydrographical Surveying. 



The glare of a powerful light may be observed accurately 

 at a distance from wliich the hght itself is below the horizon. 

 The wires of the theodolite may be seen at night by slightly 

 illuminating the object glass of the telescope through the 

 reflection of the light from a bull's-eye lantern upon a piece 

 of white cardboard held in a suitable position. 



steam 

 Cutters. 



BOATS' FITTINGS. 



It is impossible to lay down any dogmatic rules for fitting 

 boats for surveying work, as so much depends on individual 

 tastes and requirements of the locality ; but a few points may 

 be noted which have been found generally useful, and a hst 

 of articles which are always being wanted can be added as some 

 sort of guide. 



Steam cutters are, of course, the best boats for general 

 .sounding, as the engine never tires. The additional work that 

 can be done with steam cutters at command is enormous, as 

 they not only do their own work, but tow the pulling- boats 

 to and from their stations, and thereby save many hours that 

 would otherwise be spent in beating up or pulling backwards 

 and forwards to the ship morning and evening. 



A little table may conveniently be fitted in the stern-sheets 

 of a cutter, as there is plenty of room. The stern-sheet 

 canopy, which will generally have to be in place when sound- 



