Equation. 



332 IIYDROGRAPHICAL SURVEYING [chap. xtv. 



return Avire. These arrangements were entirely successful over 

 a distance of more than 4,000 miles, which is the longest 

 distance over which such work has yet been carried out. 



The indications on the tapes were not sharply defined, but 

 it was possible to detect unmistakably the first indication of 

 the movement of the syphon recorder, which was the par- 

 ticular phase selected for measurement in each case. v 

 Personal The following formulae show the effect of the observer's 

 personal equation on the results : 



A and B are the observers at X. 

 C and D are the observers at Y. 



+ & = B's personal equation on A as determined by equal 



altitude observations. 

 + c = Cs personal equation on A as determined by equal 



altitude observations. 

 + c?=D's personal equation on A as determined by equal 



altitude observations. 



A = local time of the signal at X by A's observations. 



-*^ J> J> >> JJ -^ 5) -t* S 5J 



n - V r'«j 



'-^ >> j> )> j> -•- }> '-^ " >> 



D- Y T)'s 



■*^ JJ JJ JJ JJ -"- JJ -"-^ " >> 



If X is situated to the eastward of Y, 



Meridian distance = local time at X — local time at Y. 



_A + (B+6) {C + c) + (D + d) 



~ 2 ~ 2 



(A + B)-(C + D) b-c~d 



If Y is situated to the eastward of X, 



Meridian chstance = local time at Y - local time at X. 



_{G + c) + (D + d) A + (B+6) 



2 2 



(C + D)-(A + B) b-c-d 



The amount and effect of personal equation being known 

 beforehand, it is sometimes possible to combine the observers 



