MAGNETIC OBSERVATIONS. 



91 



On the accompanying chart of iso-magnetic lines in the vicinity of Smith Strait, 

 the isogenic lines are shown by full lines ; they depend upon eleven observed 

 declinations, those at Camp Separation and Potato Camp were excluded on account 

 of instrumental defect and discordance, and Kane's determination at Van Rensse- 

 laer Harbor {D ^ 108° 12' W., June, 1854, latitude 78° 37', longitude 70° 53') 

 was admitted without correction for secular change, which is at present too imper- 

 fectly linown and is certainly less than the errors to which the observations are 

 liable. 



The following simple expression for the distribution of the magnetic declination 

 is sufficient for our case : — 



D= D^ + xA^ + yA2. cos <p 

 where 



D = resulting declination, at adopted epoch in latitude ^, longitude A. 



D^ = mean declination at epoch, in mean latitude <p^ and mean longitude /l^ 



A^ =^—^^ and A;i = ;i — 2.^ 



These eleven observations give as many equations of conditions of the form 

 = D^ — D + xA^ + yAT^ cos ^ from which x and ?/ can be eliminated by the 

 ordinary process. 



We find i»o = + 109°.97 ^„ = 78°.67 ;i„ = 72°.37 



and B = + 109°.97 + 1.61 A^ + 14.65 A;i cos ^ 



by means of which equation the isogenic lines for 105°, 110°, and 115° have been 

 located on the chart; the epoch is 1861. 



The observations are represented as follows : — 





Observed D. 



Computed D. 



Difference. 



Starr Island . . . . . 



+ 109°.'75 



+ 111°.5T 



— 1°.82 



Cairn Point 









+ 110.15 



-1-111.49 



—1.34 



Foggy Camp 









+ 106.88 



+ 109.64 



—2.76 



Camp Hawks . 









+ 115.63 



+ 113 29 



+ 2.34 



Cache on Floe . ': * 









+ 113.88 



+ 112.63 



+ 1.25 



Scouse Camp 









+ 112.10 



+ 112.59 



—0.49 



Last Camp 









+ 108.60 



+ 109.18 



—0.58 



Port Foulke 









-mi. 67 



+ 111. 2T 



+ 0.40 



Northumberland Island 









+ 106.00 



-|-10'7.42 



—1.42 



Netlik 









+ 106.82 



+ 104.27 



+ 2.55 



1 Van Rensselaer Harbor 









+ 108.20 



+ 105.64 



+ 2.56 



Probable error of any single determination + 1°.3, and of any resulting line on 

 chart + 0°.4 nearly. These lines, when prolonged in one direction, must necessarily 

 pass through the geographical pole, and in the other they extend to the magnetic 

 pole. 



