CHAP. III.] BASE BY MAST-HEAD ANGLE 71 



The vertical circle of a theodolite being only marked to 

 minutes, unless it be a much larger one than is generally avail- 

 able, is not sufficiently accurate for this. 



It is well to use two sextants to check errors, and read them 

 both on and off the arc. 



The height of mast-head above the line must be accurately 

 known to give a good result. 



Working out a right-angled triangle gives the distance 

 required. A table should be formed of the distances correspond- 

 ing to different angles of the mast-head of the ship, as this will 

 be frequently used in sounding banks. 



Two stations on shore, visible from each other with the ship Base by 

 nearly midway between them, give the best conditions for ^.ngie 

 obtaining a long base by this method. The ship's truck is from Two 

 shot up simultaneously by theodolite from each station, and 

 at the same time the angles of elevation are observed by sex- 

 tant. It is conveniently done by mast-heading a flag at regular 

 prearranged intervals, dipping ten seconds after, at which signal 

 the observations are made simultaneously. The distance 

 between the stations is determined by the mean of a number 

 of separate observations. A 30-foot bamboo lashed to the 

 mast-head is useful to give additional height. It is not neces- 

 sary for the ship to be moored or even at anchor. 



BASE BY ANGLE OF SHORT MEASURED LENGTH. 



Where the ship is not available, a base for a small plan can 

 be obtained by measuring the angle between two well-defined 

 marks placed in the ground at a carefully measured distance 

 apart, or that subtended by the ends of a long pole. 



This must also be done with the sextant or micrometer. 



If staves in the ground are used, care must be taken that they 

 are at right angles to the required base. Similarly, if a pole 

 is used, care must be taken to hold it at right angles to the 

 observer, which can be ensured, either by having a pointer 

 nailed on to the centre of the pole projecting at right angles, 

 and which must be directed towards the observer by the man 

 holding the pole in both hands horizontal, or by simply waving 

 the pole, held in this position, backwards and forwards gently. 



