26 



HYDROGRAPHICAL SURVEYING 



riiAT. I. 



On the 



Circle. 



Sensitive 

 Circles. 



General 

 Rules. 



could move the centre of the instrument considerably, without 

 materially affecting the coincidence of the legs with the three 

 points. 



When X is so placed as to fall on a circle passing through 

 the three points ABC, there will be no intersection whatever, 

 as the two circles will coincide ; and we cannot tell where we 

 are on the circumference of this practically single circle. 



The nearer, therefore, we are to being on a circle, whose cir- 

 cumference will include all the three points and our own 

 position, the worse will be what is technically called the 

 fix, and this must always be guarded against in selecting 

 objects to observe. 



When one object is farther from us than the central one, 

 we shall, as a rule, have a good fix ; but when the central 

 object is the farthest, the two circles will begin to make a bad 

 intersection. 



A circle is " sensitive " when the angle between the two 

 objects responds readily to any small movement of the observer 

 towards or away from the centre of the circle passing through 

 the observer's position and the objects. This is notably the 

 case when one object is very near to the observer and the other 

 very distant ; but not so when both objects are distant. Speak- 

 ing generally, the sensibility of an angle depends upon the rela- 

 tive distances of the two objects from the observer, as well as 

 on the absolute distance of the nearer of the two. 



The more rectangular the angle at which the circles inter- 

 sect each other, and the more " sensitive " they are, the 

 better will be the fix ; one condition is useless without the 

 other. 



There is nothing in the whole range of surveying that requires 

 so much attention and knowledge as the fix, and many are 

 the errors which have crept into surveys from disregard of its 

 conditions. 



When moving along, as when sounding, and fixing from time 

 to time, if both angles change slowly, the fix will be bad, for 

 we must be moving nearly along the circumferences of both 

 circles, and they must therefore nearly coincide. 



In plotting the angles with the station pointer, the fix will 

 be good if a very slight movement of the centre of the instru- 

 ment throws one or more of the points away from the leg ; but 



