Surveying and Levelling, 169 



Or in words, the amount of change of zero, or the starting points of A will 



be obtained by dividing 360° by the product of the number of repetitions 



multiplied by the number of verniers, or even more simply, by dividing the 



interval between each vernier by the number of repetitions, the quotient 



will be the change of each in degrees. Let the required number of rc- 



360° 



petitions be fouVf the number of verniers three, then = 30°, the 



\z 



change. Hence the starting points of A will be 0% 30°, 60% and 90", by 

 which means the whole circle is equally employed in measuring the re- 

 quired angle, and the errors of excess and defect will, as I have often 

 found by experience, even by a moderate sized circle, with a great de- 

 gree of probability, neutralize each other. 



Table I. To convert feet on the Terrestrial Spheroid into Seconds of Arc. 



Table II. To reduce a Base at the level of the Sea to any altitude above it- 

 and conversely, S^c. 



