CHAP. I.] THEODOLITE 21 



Angles to main stations, however, will be very likely re- Repeating 

 quired to enter into the calculation, and the correctness of the "^ ®^* 

 plotting will any way depend on them. These must therefore 

 be repeated, the number of times varying according to the 

 degree of accurac}^ required. 



One method of repeating angles is thus given in Heather, 

 somewhat altered. 



Having taken the first measurement, loosen the clamp of First 

 the lower plate, turn the theodoUte bodily round until the Method, 

 telescope is directed upon the zero-object, and again clamping 

 the instrument, perfect the bisection of the zero by the cross- 

 wires by means of the slow-motion screw on the neck of the 

 instrument. The index of the vernier, together with the coin- 

 cident division of the limb, will thus have been brought from 

 the position in which it was when the telescope pointed at 

 the object to be measured, round to the previous position of 

 the 360°. 



Now release the upper or vernier plate (looking again at the 

 vernier first to see it has not been moved), turn it until the 

 telescope is again directed towards the object, clamp and per- 

 fect the bisection by the tangent screw moving the upper plate. 

 The reading now on the vernier will be twice that formerly 

 read off, or nearly so, and will be entered in the book under 

 the former observation. This process can be repeated as often 

 as required. The mean angle can be obtained by dividing the 

 last reading (increased by as many 360° as the plate has 

 revolved) by the number of observations, but it is better for 

 our purposes to put down each individual reading. The differ- 

 ence between every two consecutive readings will give a value 

 for the angle, and we can then see how they agree with one 

 another. An example of this kind of repeating is given on 

 p. 85. 



The above method is perhaps the most accurate ; but, when Second 

 many angles are to be taken, requires much time, and we shall ® ° ' 

 arrive at a conclusion quite near enough for any hydrographical 

 triangulation by taking all the angles in succession with the 

 vernier set to 360°, and then, changing the degree of the zero 

 to some even submultiple of 360°, as 90°, 180°, etc., take all 

 objects again, repeating thus as often as necessary ; this will 

 be found much quicker. 360°, 120°, and 240° divide the arc 



