86 HYDROGRAPHICAL SURVEYING [chap. iv. 



T? July 4th, 1881, at Pagoda a ® Patero A, Theod. 77. 



Compari- 

 son of 

 Methods. 



The first of these forms is adapted for the observation of 

 main angles by repeating round and round singly ; which 

 is done when a solitary angle is required to be observed 

 accurately, but to obtain great accuracy the reading should be 

 repeated right round the arc. 



The second is for ordinary main angles. This method saves 

 much time when there are a number of angles required, and 

 is as correct as is generally necessary. 



The weak point of the first method is that the zero cannot 

 be referred to, but, as only one angle is taken each time, a 

 theodolite must be very much out of order to introduce error. 



If the angle to be observed is small, this method will not 

 answer the purpose, as the theodolite will only be rotated 

 through a small part of the circle, unless an inconveniently 

 large number of repetitions be made. 



The weak point of the second method is that any shght 

 error in setting or reading the zero affects every station 

 observed ; whereas in the other, the vernier being once set at 

 the commencement, is afterwards read only. 



By either method the observer will see if his different obser- 

 vations of each angle are agreeing together, and can take more 

 if requisite. 



In all observations of angles with the theodolite (except the 

 the Zero, ^asc referred to above), the zero must be looked at from time 

 to time, and invariably at the conclusion of the set of angles, 

 to make certain that the direction of the instrument has not 

 changed by any unnoticed touch or shock. On every occasion 

 of doing this it must be noted in the book, so as to know, in 



Verifying 



