32 HYDROGRAPHICAL SURVEYING [chap. i. 



time to time examined by its means. Screwing on the length- 

 ening logs, and placing the station pointer on this sheet, with 

 the nick in the centre of the arc corresponding exactly with 

 the prick in the centre of your testing circle, and putting 

 weights on the central part of the station pointer, each 

 leg can be in turn moved to correspond with the ruled 

 10° lines, and the reading of the vernier compared. The 

 error at each 10° should be written on a small piece of 

 paper in the form of a table, and pasted on the inside of the 

 box. 



If the legs of the instrument are exactly centred, the readings 

 will either be correct, or the same amount in error all round, 

 for each leg ; but as this is a degree of delicacy rarely attained, 

 it will usually be found that the error varies for different posi- 

 tions of the leg. The verniers should be set to minimise the 

 errors between 0° and 90°, which is the amount of angle most 

 used in actual work. 



The chamfered edge of the leg and lengthening piece should 

 correspond exactly with the line in all its length ; if it does not, 

 it is also a result of bad centring or bad fitting on of the length- 

 ening piece, but a good instrument should not have this error 

 in any appreciable extent. 



It need scarcely be added that if the instrument is found very 

 badly centred, it should be returned to the maker, or not be 

 chosen if buying ; but when an instrument is sent to the other 

 end of the world, you may have to make the best of it, and 

 registering all the errors on the table, be careful to apply them 

 when using the instrument. 

 Discretion The necessity for applying a small error depends upon cir- 

 as to cumstances, as, in some cases, the position of the points used 

 Errors. will admit of a difiPerence of several minutes in the angle, with- 

 out any appreciable alteration of the position of the observer ; 

 in others, it is necessary to be exact. As the surveyor 

 gains experience, he will learn when to apply the error, and 

 when not. At the commencement, he must always apply the 

 error. 

 Caution It may here be noted, that, if the points used to fix by are 

 of station ^^^ correctly placed on the chart, the station pointer will not 

 Pointer, indicate anything wrong, unless a third, or " check " angle, be 

 taken and plotted. This must always be remembered in using 



