An Underground Traverse. 



417 



The date of survey is recorded. The instrument is clamped at 

 any point of circle, thus avoiding wear in the clamps, etc., when 

 always set at zero. If the booking be consecutive with the procedure 

 of observation, there can be no mistake about which is the angle 

 measured in the right direction. Thus, the reading at 931, being 

 the foresight, is subsequent to the travel of the telescope from the 

 reading at 929, being the backsight. vSubtracting the first reading 

 from the second gives the angle measured in the right-hand direction. 

 Now, to check this, the telescope is transited, and the operations 

 repeated. But rather than check this angle, measured in the right 

 direction by again subtracting the first reading from the second, the 

 writer prefers to take out the angle measured in the left direction, 

 i.e., subtracting the second reading from the first (vide calculation), 

 and thus ensure that the sum of the two shall equal approximately 

 360 degrees. All the numerical work that is required at the instru- 

 ment when observing is mentally to determine that the readings, 

 when the telescope is transited, are 180 degrees from the first 

 readings. 



II. CALCULATION. 



(square section paper) 



21/5/06. 



27 



