.EXPEDITION TO POINT HAKKOW, ALASKA. 449 



'/ // 

 "To correct 1'or changes in the value of . for change "I temperature <if magnet let '/= 



change of magnetic moment of magnet corresponding to a change, of 1 Falir.. we have, ilic correc- 

 tion </ (t t e ) where t,,= normal temperature adopted and f=any other temperature. The value of 

 q may be found by a series of observations of oscillations at high and low temperatures, tin; mag- 

 net being .suspended as in the unitilar magnetometer. Putting A= cot r we have 



'I tf 



Jf = A- ( ) + </ (/ /) 



(he value of fc may be about .(MM) J."> and it should be so arranged, by varying the distance of the 

 threads, that (he least integer reading of the scale should indicate about to part of the 



MUM* JIHMJO 



horizontal force. The observed variation in the horizontal component of the magnetic force will 

 be true only in case the magnetic moment of the suspended magnet remains unchanged during 

 the time of observations, but as every magnet gradually loses magnetism a further correction for 

 loss of magnetic moment is needed. This may be determined by comparing differences of values 

 of horizontal force as determined by means of the unifilar magnetometer at certain times (and 

 after long intervals) with a series of corresponding readings of the differential instrument. The 

 magnet being an old one, it seems best to examine and readjust the bifilar at the end of each year 

 or ofteuer in case of necessity. 



"The north end of the magnet may be turned either to the right or left of the meridian, but 

 it will be desirable to choose that side which will make inert nuintj horizontal force correspond to 

 increasing scale readings. 



"The principal adjustments of the instrument may be summed up as follows: 



"Level; suspend magnet as unitilar; focus telescope ; place scale horizontal and adjust light 

 for distinct vision; take torsion out of suspension ; put plan*' of detorsion in magnetic meridian; 

 determine axis of collimator; determine scale value or value of one division in minutes of arc; 

 point on axis and note corresponding scale reading of magnetic meridian; take off unifilar and 

 substitute bifilar tube; place plane of bifilar suspension in magnetic meridian, point on axis and 

 read torsion circle ; test this by turning telescope 180 in azimuth and bringing the magnet in the re- 

 versed position, north end to the south, and read torsion scale; if it reads as before, the plane of 

 threads was tnily in the magnetic meridian ; repeat adjustment if necessary; turn telescope 'M 

 or into the magnetic prime-vertical and turn in the sinnr ilirection the torsion circle until the axis 

 of the collimator appears pointed in telescope; read the torsion circle, it will be !Kl+ / from the 

 meridian value ; compute the value of /. and alter the distance of threads by turning the screw 

 itntil a satisfactory value for Jc is found. 



"The observers will remember that at Point Barrow the horizontal force is about one-half of 

 what it is at Washington. They may also consult Lloyd's Treatise on Magnetism (London. 1ST 1).'' 



With reference to co-operation with the Polar Commission during the ec0nd year of occupation 



of the Point Barrow station, directions were given by you, May 23, 18812, to prepare the old Brooke 

 magnetographs for immediate service. These instruments had been used for many years, first 

 at Key West, Fla.,* and lately at Madison, Wis., and required thorough overhauling; moreover, 

 photographic registration being out of the question in the Polar regions, they were changed and 

 remounted according to a plan devised by me, for direct eye-observations. By extra exertion, 

 with the assistance of Fauth & Co., instrument makers, and W. Suess, mechanician, this was 

 expeditiously done, and the instruments left Washington .June 14,. 1882. 



The following memorandum was handed to the relief party before starting for Point Harrow: 



"MAY 20. 1SS2. 



"The magnetic instruments intended for Point Barrow will be the modified Brooke Magnet- 

 ometer, viz, declinometer, bifilar or horizontal force magnetometer, and Lloyd's balance or vertical 



* For a description see Coo( Survcx I.Yn>vt I'm- l!-r>l>, A|>i>ri,i',ix Ni. ::<;. or Ilio nri^iiml ]>}} in 1'lii!. Trans. 

 Hoy. Snr. 1M7. |i:nt 1, ' On tin- jmfoimitir rrj;iMr;itiiJii of iii:i}',i:rlrn.i d i.*. A i . . \-\ ].!;.: ci-;r;ij>ln . J!\ Cli;nlos B:cx>k<>. 

 .Tnur, 1M(,." 



II. Ex. 44 57 



