CHAP, v.] FIXING MARKS 1G7 



A heliostat is often invaluable. In hazy weather, and when Use of 

 the shooting-up station is distant especially, a flash will be seen ^^^lostab 

 when neither mark, nor boat, nor anytliing to direct where to 

 look for the mark, will be visible. The officer shooting up 

 should also return the flash, to show he sees the station, as well 

 as give a well-defined object to get the angles to. 



Of course, circumstances may not render this system ad- 

 visable, but it is here suggested as having worked very well in 

 many places, a long extent of coast being " marked " and all 

 marks fixed in a short time. 



Frequently the minor marks must be fixed by angles from Triangu- 

 the ship, or a boat at anchor, as on a straight coast where ^**i°/'- 

 nothing behind can be seen from the marks. When this is Marks by 

 necessary it will often be also necessary to carry on the main ^^^^' 

 triangulation as well by means of ships and boats, so that a 

 description of one serves for the other. 



The ship, anchored short, or moored if necessary, should be 

 shot up from one or more shore stations. If the angles taken 

 from the ship are indispensable to fix her own position, try 

 calculating the back angles from other objects first, and lay 

 them off as cuts to the position, as if they agree it will be the 

 most satisfactory manner ; but often back angles, calculated 

 from sextant angles, will not be correct enough to give a good 

 intersection, especially if the points are distant. In this case 

 let all the angles taken at the ship or boat be plotted on tracing- 

 paper as before described, and the position pricked through on 

 the guiding line from the shore station. A signal should be 

 made when the angle to the ship is to be observed, and the 

 angles from the ship taken at the same time. 



The ship angles should be observed from the fore part of the 

 ship, and frequently the foretop will be found the best place. 

 Whatever spot is used it must, of course, be arranged before- 

 hand, so that the observer's exact position on board may be 

 taken from the shore station. 



From the ship the main angles — that is, the angles to the Taking 

 positions already plotted — which are to be taken for the pur- f^^^tfh- 

 pose of fixing the ship, must be observed first, using some well- 

 defined station as zero, and measuring all the main angles 

 from this with the sextant. Some other station must be 

 chosen as the zero with which to measure the angles to the 



