EXPEDITION TO POINT HARROW, ALASKA. 



Resulting monthly meanx of tltr mnrinclic declination (it l!</l<t<nui<: 



459 



The value 35 27'.2 for the epoch Marcli 1, 1883, is i)referrcd to tbe value deduced above for 

 the epoch June 1, 1883. The corresponding value of the Brooke deeliuometer reading is 484.7 



Respecting the annual change of the declination due to the secular variation, we know from 

 t lie general discussion of the secular variation, Appendix Xo. 12, Coast and Geodetic Survey Report 

 for 1S82, that the eastern declination in Alaska is now diminishing. The expression for the secular 

 variation at the two stations nearest to Point Barrow, viz, Port Clarence, in </; = G,"P 17' and 

 A = 160 19' west of Greenwich, and Chaniisso Island, in </> = C6 13'.:! and A = 101 48'. 7 west of 

 Greenwich, give for the annual change in 18SO and 1885 the values -f 10'.3 and + 1T.3 for Port 

 Clarence, and +10'.7 and + 12'.0 for Chamisso Island, and we have to expect a greater value at 

 Point Barrow. Captain Maguire determined the declination at that place in 1853. and found 

 HI 21', or. when reduced to Uglaamie, about 40 Oti'. which, compared with our value above, 

 gives almost exactly a diminution of 4i between 1853 and 1883. It is known, from the other sta- 

 tions, that this declination has not passed through a maximum within the last thirty years, but lias 

 diminished gradually, with an accelerating rate. For uniform speed, the annual change would be 

 + Id' : it is, therefore, probably near -f 15'. The absolute measures September, 1882, to August, 

 l.s.s;; would give the value -f 28'. 4, which is known to be greatly in excess, and if we fall back on 

 the differential series, we obtain a value but a trifle less, and undoubtedly affected by torsion in 

 the suspension skein of the declinometer, which was never re-examined after the first adjustment 

 had been made. Omitting the readings between March and April, when the torsion was most 

 pronounced, a discussion of the 5 monthly means, November, 1SS2, to February, 18S3, inclusive, 

 give a monthly change ni = 0'.97, and a discussion of the 4 monthly means for May, -June, July, 

 August, 1883, gives m = T.15, but if April be included m = l'.!>2, mean = T.53; mean of 

 first and last value 1'.25, hence annual change + 15'. 0, which is adopted as the most probable 



value. 



j 



ABSOLUTE MEASURES RESULTS OF THE MAGNETIC DIl'. 



The observations were made with the Kew Dip Circle,* L. Casella (London), No. 4370, or Coast 

 and Geodetic Survey, No. 23. It remained mounted on its pier in the small magnetic observatory 

 during the stay at I'glaamie. The instrument left Washington .lune 23, 1881, and was returned 

 January 12, 1SS4, only sustaining the breakage of one of the dipping needles. Test observations 

 made by Sergeant Maxh'eld at Washington in January and February, 1884, on four days, gave 

 very satisfactory results. (See results for intensity.) 



Observations were generally made on three days each month. The series commences with 

 November 30, l.ssi. and ends with August 14. 1883. It does not appear that there is any appre- 

 ciable difference in the results by needles 1 and 2; they are therefore combined indiscriminately. 

 The following monthly means are made up from the individual results contained in Appendix No. 2, 

 and they are here arranged with a view of deducing, if practicable, from the monthly values, taken 

 at an interval of a year, a value for the annual change of the dip, independent of any annual 

 variation. 



* l-'inre<l in Const and 



Suivr\ l,Y[Mut l'r 1--!, 



x No. *, 1'latr N'<>. :!T. 



