MAGNETIC SURVEY OF GREAT BRITAIN. 



183 



With respect to these observations. Captain Ross observes : 

 "The I'eadings of R, L 4 at Dublin, with the letters on the needle 

 to the face of the instrument, gave 5° greater when facing the 

 east, and .5° less when facing the west, than the mean of similar 

 facings with the needle reversed on its axle. I therefore thought 

 that the axle had got some bend, and was totally ruined; and 

 accordingly used R L 3 always in future. But at Londonderry 

 I had some spare time, and thought I would try and find out the 

 cause of this error, for I was sure it had sustained an injury. 



"At Londonderry the mean of the readings E. and W., with 

 the letters to the face of the instrument, was 2| degrees less 

 than the mean of similar readings with the needle reversed on 

 its axle. I therefore believe that some considerable irregularity 

 of the axle, about the point where the needle (with its letters 

 to face of instrument) should rest at about —7°; has occasioned 

 this error; and the circumstance of the Dublin observation 

 coming out right, is merely aftiidental. In all other parts of the 

 axle that I have tried, its readings agree very nearly with each 

 other." 



Under the circumstances above detailed, it seems necessary 

 to reject the observations with RL 4 at Dublin and Londonderry. 



The following Table contains the compxited results of the 

 foregoing observations, and the latitudes and longitudes of the 

 stations. Li making the computation, no correction for tempe- 

 rature has been applied to the results of R L 3 ; the logarithmic 

 correction of R L 4 is -000024.* 



See pages 157 and 158. 

 N 2 



