EXPEDITION TO POINT BARROW, ALASKA. 453 



the case <>!' ;i free declination magnet, observing that the .sides which were vertical when on its 

 bearings will now be horizontal. The moment of inertia will be, the same as before. Let 'l\ thti 

 lime (if;i horizontal oscillation, then 



,5 V 1\~ 'l\ l 



.* = ,.,'/ cot dip. <;= ,.,,<,'- cm a 



where a SK dip. For one. linear unit nf scale and / units of distance lo mirror the value- of <.' sa _ 



The dip is ID be determined by means of the dip circle. For a particular scale value, '/', having 

 been determined, we alter the position of the center of gravity by the adjusting screw, until by 

 trial the desired value of T is produced. The scale value may also be ascertained by means of 

 deflections, the ma-net being lirst in a horizontal, next in a vertical position. (Seep. 65 of 2d 

 part of Bulletin, St. Petersburg, 18S2.)* 



The temperature compensation originally with the Brooke balancing magnetometer consisted 

 of a glass thermometer tube tilled with mercury. This lias been removed, and a brass arm wan 

 substituted, as in the Adie instrument. The compensation operates as follows: Suppose the tem- 

 perature is rising: the magnetic energy of the hoii/ontal magnet will diminish, and gravity will 

 consequently .pull the south or unmarked end of the, magnet down and thus elevate the marked 

 end, but this is counteracted and the balance restored by the expansion of the brass arm which is 

 directed to or on the same side as the marked end; the diminution of magnetic moment is thus 

 counteracted by the increased leverage of the extended brass arm. 



"Increasing scale, readings should correspond to increasing vertical magnetic force or to a 

 movement of the north <-n<l of the magnet sloicnicitrrl. The instrument is placed under cover of 

 thick plate-glass. 



"Referring to the diagram of the magnetic observatory containing the modified Brooke differ- 

 ential or variation instruments, it will be seen that the north seeking or marked ends of the mag- 

 nets turn all to the inside or toward the telescope-pier. The directions in which the scale-numbers 

 in. -rease are also there indicated. 



"Time beini; wanting for an accurate mechanical compensation of the force magnetometers, it 

 is the intention that only the greater part of the change should be so compensated and collections 

 applied for the remaind'-r. For this purpose thermometers are inserted, which are to be read in 

 connection with the, scales. The data for outstanding temperature correction will be, had from the 

 ordinary hourly observations." 



The Point "Barrow party was also put in possession of the resolutions adopted at the third 

 session of the, International Polar Conference, held at St. Petersburg, August, 1881. From this 

 publication the following notes were taken: 



'The differential magnetic observations for changes of declination, hori/.ontal and vertical 

 components of the earth's magnetic force, are to be made hourly and continuously, commencing as 

 soon as possible on or after August 1. 1882, and closing as late as practicable before or on Sep- 

 tember 1, iss:;. 



"These hourly olwri-iitioitx may be made either with reference to l<n-l time or with reference 

 to any other meridian. [The full hours of local mean time are recommended, and the instruments 

 arc to be read in the order, bililar li minutes before and after, unitilar 1 minute before and 1 min- 

 ute after, and balance magnetometer minute before and i minute after each full hour.J 



"Term- day observations. Term-days are the 1st and 15th of rttch month (excepting January 

 1, when January 2 will be taken). The differential instruments on term-days are observed evcry 

 5 minutes throughout the 21 hours, and strictly according to (!iitHni/i->i wi-mi ciril time, beginning 

 with O' 1 ()'" (or midnight, < lot tingen.) The three instruments will be read as rapidly as possible, 

 one after another, in the order given above, the declinometer being read at the exact full tilth 

 minute. 



".\iliHHniiiil obtervationt to be made on term-days during out- hour are specified below. Declina- 



* II' ; :ui^lr which the line joining (he ccntrrs of gravity :ni<l "f motion makes with the axis of the 



'/" V I 



\vi- have tan ; tan <i = ,.,'.: also .. ~- < : "i '' and .since in 0111 caw i; ~ '.III' , form n la (:!) ot')i. ('.I! changes to8T= R . V, 



-sr / 



ln-ni-c, _ ^ ,.,,! f ; , as aliove. 



