MAGNETIC OCSERYATIONS. 



83 



In the above record I have given the circle reading in those cases only when the 

 circle had been shifted between the two honrs of record, its reading from day to 

 day being otherwise of no consequence. If we take the difference each day of the 

 tabular numbers, we find, from 104 days, the average difference 4.42 divisions, or 

 45', by which quantity the north end of the needle moved easterly between 2 and 

 10 P. M. By the preceding diurnal curve we must add 1' before 2 P. M., and add 

 4' after 10 P. M. in order to get to the extreme range, which is therefore 60', a 

 value preferable to that given before. 



At Philadelphia the ratio of the diurnal range in winter, to that of the whole 

 year, is as 5.6 to 7.9, hence applying the same ratio to Port Foulke, we find the 

 probable diurnal amplitude of the declination, on the average throughout the year 

 and for an epoch of its greatest value in the ten or eleven year cycle, to be 1° 10'. 



ABSOLUTE DETERMINATIONS. 



Observations and Results of Magnetic Declinations. 



The declination observations made in connection with the survey of the west 

 coast of Smith Sound and Kennedy Channel, in the spring of 1861, will be given 

 first, next those observed in Smith Strait, and last those determined in North Green- 

 land. There are 14 stations in all. 



An approximate correction for diurnal variation was applied to refer the observed 

 declination to the mean declination of the day ; this correction was derived from 

 the mean diurnal progression as found at Port Foullie and Van Rensselaer Harbor. 



Cairn Point, Smith Strait. 

 Observations of magnetic declination, April 9, 18G1. S. J. McCormick, observer. 

 Double altitudes and bearing of the sun. 

 Sextant: 2© 



25° 14' 

 25 02 



24 53 



Mean, 

 Index correction. 



Refraction — par., 

 Semi-diameter, 



Observed altitude, h. 12 44 



Latitude, 

 Lonffitude, 



^= 68° SO'.S 

 ,,=4'' 51" 56" 



O's decl'n, S = 1° 49' 15" 

 Hour angle, t = 4'^ 15" 14' 



j/=n° 17' 11" 



Azimuth, A = 65° 42' 



d) Mag. bearing S. 1T6 00 W. 



sin h — sin $ sw S 



cos t cos S 



Put tg M 

 then cos A 



_ tgS 

 cos t 



tgh 



cotg (ij) — 71/) 



Mag. decl'n, -|- 110 08 at 4^ 



Observation of magnetic declination, April 12, 1861. S. J. McCormick, observer. 



Bearing of the sun at noon N. 70° W. 



Hence magnetic declination . . . . -f 110° 0' 



