508 HYDRO GRAPHICAL SURVEYING [chap. xxi. 



lengths of planldng being separately distinguished, the beacons 

 could be readily prepared and dismantled in the boats, to be 

 used on future occasions as required. 



It is not necessary here to mike any special reference to any 

 other form of beacons, which may take the form of targets or 

 mark buoys, such as cross-planks or single cask, each surmount- 

 ing a staff and flag, or Dan buoys, all of which can be used with 

 advantage within then* known limits of vision and de]Dth of 

 water. 



SECTION XXV. 



obtaininCt the distance between any selected 

 positions near the extremes of the ship. 



Where no rehable fitted drawing of the ship on a sufficiently 

 large scale is available it may be sometimes necessary to deter- 

 mine this. This is best done by means of a distance measured 

 on shore made at a convenient distance from the ship when 

 secured to a neighbouring wharf, etc. 



Two intervisible terminal points, commanding a view of 

 both ends of the ship, should be selected on shore near abreast 

 the ends of the ship, the intervening ground being adapted 

 for a rehable measurement to be made between these points, 

 which are carefully marked, and in such a manner that they 

 can be used as reference-marks, the one from the other, with a 

 theodohte when placed at each terminal point. 



Theodolites are set up exactly over these spots, and horizontal 

 angles measured from each as a reference point to the selected 

 points near the extremes of the ship, Avhere vertical battens are 

 erected and then' positions accurately marked on the deck. 



Two observers should be employed on shore, and simultaneous 

 intersections made from each end when the ship is motionless, 

 for obviously any movement of the ship during the interval 

 between the intersections would vitiate the result. 



The necessary computations can then be made for determining 

 the horizontal distance between the points intersected on board 

 the ship, the receiving angles in each case approximating to a 

 right angle. 



We may require to determine the horizontal distance between, 

 say, the centre of the trucks of the ensign and jackstafE or a 



