412 Report S.A.A. Advancement of Science. 



A footnote is added : " To guard against an error in adding or 

 subtracting i8o degrees when changing directions from forward to 

 back, use the following check. When a sheet of paper is full, i.e., 

 has 8 points calculated on it, write down the first direction, and add 

 to it the sum of all the positive angles, subtract a multiple of 360 

 degrees, and add or subtract 180 degrees; result should be the 

 direction of the last line at bottom of page." 



Thus : Check at bottom of fage. 



Direction at top + sum of all angles + or - 180 degs. - direction 



at bottom. 



I27-33-30 

 I745r o 

 I69-56- 

 191 ^ZSO 

 173- 7- 

 I93-59- 

 I62I3- 

 I8240- 

 I9I58-30 



1568- 620 



1440- =4x360 degs. 



128 

 + 180 



308' 620 



(This check is done on any scrap of paper and is not filed). 



He applies a further check when starting a fresh page, thus : 



Check, on additions of 



Co-ordinate Distances. (Northings, westings, etc.) 

 Totals. — Co-ordinates at top of page = co-ordinates at bottom of page. 



+ 12933-64 Bottom of page +4743392 

 (This check is done on any scrap of paper and is not filed.) 



Similar to (2), this is systematic, but fails in exactly the same 

 particulars, viz., the mechanical check of addition and subtraction. 



