56 A MANUAL OF TOPOGRAPHIC METHODS. 



be tightly clamped. Looseness of the foot screws and tripod is a common 

 source of error, especially with small instruments. 



The alidade, or part of the instrument carrying the telescope and 

 verniers or microscopes, should move freely on the vertical axis. Clamps 

 should likewise move freely when loosened. Whenever either of these 

 moves tightly, the instrument needs cleaning, oiling, or adjusting. 



The observer should always have a definite preliminary knowledge of 

 the objects or signals observed. The lack of it may lead to serious error 

 and entail cost much in excess of that involved in getting such knowledge. 



Great care should be taken to insure correctness in the degrees and 

 minutes of an observed angle. The removal of an ambiguity in them is 

 sometimes a troublesome or expensive task. 



The errors to which measured angles are subject may be divided into 

 two classes — viz., first, those dependent on the instrument used, or instru- 

 mental errors; and second, those arising from all other sources, which, for 

 the sake of distinction, may be called extra-instrumental errors. 



The best instruments are more or less defective, and all adjustments 

 on which precision depends are liable to derangement. Hence the general 

 practice of arranging observations in such a manner that the errors due to 

 instrumental defects will be eliminated in the end results. The principal 

 errors of this kind and the methods of avoiding their effects are enumerated 

 below. 



Measurements made with a graduated circle are subject to certain sys- 

 tematic errors commonly called periodic. Certain of these errors are always 

 eliminated in the mean (or sum) of the readings of the equidistant verniers 

 or microscopes, and both of the latter should be read with equal care in 

 precise work. Certain other errors of this class are not eliminated in the 

 mean of the microscope readings, and these only need consideration. Their 

 effect on the mean of all the measures of an angle may be rendered insig- 

 nificant by making the number of individual measures with the circles in 



opaO 



each of n equidistant positions separated by an interval equal to — — where 



1 ' ' mil . 



in is the number of equidistant verniers or microscopes. Thus, if mz=2, 



180° 



the circle should be shifted after each measure by an amount equal to 



n ' 



