ADJUSTMENT BY LEAST SQUARES 179 



shown under the respective columns in Table VIII. The approximations 

 to m obtained by summation m' -\-m" -\-m"' -{■m}"' are next given. The 

 whole process for 80 points took 15 hours, an average of just over a 

 quarter of an hour for each of the 56 gravity stations. 



The adjusted link-increment values are easily obtained from the 

 calculated w's by using equation (i), and the adjusted gravity value 

 of each station relative to any one taken as datum can then easily be 

 derived, and the isogams can be drawn. 



A check on the accuracy of reduction is obtained by computing 

 the residual excess for each triangle. From equations (i) and (2) it 

 is easy to see that the residual excess d' is given by the formula 



dr = dr -\- 2Wa — 2>''^r , 



which ought to be zero if the values m are accurately computed. The 

 actual residual excesses which remained when the calculation was 

 stopped at m^"' are given in Table VIII, column </', and are seen to be 

 below the agreed limit of error of 10 units. 



839 



