188 



REPORT — 1859. 

 Table XVII. 



Station. 



Glenraorven 

 Balmacarra 

 Kyleakin ., 

 Stornoway.. 



Callinish .. 



Cross 



Loch Inver 



Durness 



Thurso 



Lerwick 



Wick 



Golspie 



We see from this Table that though C (which is equal to twice the angle 

 by which the mirror is out in adjustment), as exhibited in the third column, 

 is somewhat large, yet the correction to be applied to the actual observations, 

 denoted by c!, is generally very small, the only exception being Glenmorven, 

 where the altitude of the sun was high at the time of the azimuth observation. 



It has been mentioned that there were two stations after Bridgend at which 

 no back observations were taken. One of these was Port Ellen, the next 

 station after Bridgend, but as the altitude of the sun was high when the azi- 

 muth observation was taken there, it has not been thought advisable to apply 

 a correction proceeding upon an assumed value of C. The other station was 

 Kirkwall, which occurs in point of time between Lerwick and Wick. The 

 value of C for Lerwick is 41''2, and for Wick it is 43' - 0. If we assume the 

 mean of these, or 42'* 1 as the value for Kirkwall, we find 5= + 2 f, 2, which 

 value has accordingly been adopted. 



Table XVIII. (p. 189) exhibits the declinations for 1858, corrected for error 

 of mirror, and reduced to 1st January, 1858. 



If we now take all the declinations, with the exception of that for Glen- 

 morven, which seems to be influenced by local attraction, we obtain by the 

 method of least squares u, or the angle which the isogonic lines make 

 with the meridian = — 20° 58'-3, or their direction is from N. 20° 58'-3E. 

 to S. 20° 5 8' -3 W. ; r, or the increase of the declination in a direction per- 

 pendicular to the isogonic lines, =sl'*465 for each geographical mile ; and d, 

 or the declination at the central station, lat. 56° 54' N., long. 4° 14' W.= 

 25° 53' - 6. The isogonic lines are exhibited in a map (Plate 7) appended to this 

 report. 



In Table XIX. (page 190) the observed and calculated declinations are 

 compared together. 



If we now divide the stations, as before, into two groups, the first comprising 

 trap and granite, and the second every other formation, we shall find the mean 

 probable error for the former group=24' - 8, and that for the latter=ll'T. 



If we examine Tables VI., XI., XIX., in which the difference between the 

 observed and calculated magnetic elements is given for the different stations 

 arranged in the order of observation, we shall, I think, perceive that stations 

 similarly affected with regard to sign are in many cases grouped together. 

 But, from the principle of arrangement adopted, the members of any such 

 group denote stations at which the observations were consecutive with respect 

 to time ; so that such stations cannot be very far apart with respect to geogra- 

 phical position. 



