362 HYDROGRAPHICAL SURVEYING [chap. xv. 



In the mean of the total errors the deviation will be elimi- 

 nated, and the result is the variation. 



A full report of the observations should be transmitted home. 



Shore Shore variations are also of value, when taken on ground free 



tions^for fi'^HTt suspicion of local attraction, for the determination of the 



Variation, true variation, and, in other cases, for the information they 



afford on the amount of the local attraction as obtained by 



comparisons with the variation found from sea observations in 



the vicinity — a point of much interest, and often of practical 



importance. 



The requirements for a good shore variation, that the Hydro- 

 graphic Office can put confidence in, are as follows : 



1. The true bearing of different points (about six) as equally 

 distributed as possible round the circle, whose centre is the 

 observation spot, must be well and accurately observed with 

 a theodolite. 



2. Bearings of all these points must be taken by the compass 

 from the observation spot. 



3. Different sets of observations must be made with different 

 pivots and with different cards. 



4. The spot on which the observation is made should be free 

 from every suspicion of any iron in the vicinity, and the nature 

 of the rock, or whatever the formation may be near the observa- 

 tion spot, should be mentioned in the return transmitted home. 



Points 1 and 2 are necessary precautions against the errors 

 of the card, caused either by bad graduation or from accidental 

 bending of the card. In ascertaining the true bearings, it will 

 only be necessary to observe one object, when theodolite angles 

 to the others will give their difference of bearing. 



As regards No. 3, all compass cards have an error caused by 

 inaccurate affixing of the magnetic needles, and it is necessary 

 to multiply observations, and make certain the card is working 

 properly. 

 Variation Shore observations should be obtained at stations where the 

 ^t*offi^*^ variation is already well kno\\'n, when opportunity offers, as 

 these wiU enable the Office to calculate the change of variation. 



An example of observation for variation is appended. 



Although the variation is here deduced to show the method, 

 this would not be done in forwarding these observations to the 

 Admiralty, as there are certain card errors to be applied first. 



