482 EXPEDITION TO POINT BAEROW, ALASKA. 



of disturbances separated was between and of the whole number; but it should be remarked 

 here that at that time we were approaching an epoch of a sun-spot minimum, whereas at present 

 we have just passed through a sun-spot maximum, during which the disturbances are greater. 



It has been noticed that a limit adopted for a station in low magnetic latitude will not servo 

 to deduce a limit for a station in high magnetic latitude when having regard only to the supposition 

 that the limits are inversely proportional to the magnitude of the horizontal components of their 

 respective magnetic intensities ; the disturbances appear to increase in greater ratio as we approach 

 the magnetic polar regions.* 



The further discussion of the differential observations must be deferred until a decision has 

 been reached by the fourth international polar conference (which is to meet shortly at Vienna) 

 respecting the limit of recognition of disturbances. [April 5, 1884.] 



THE BIFILAR MAGNETOMETER. 



The length of 1 division of the scale is 1 millimeter, the radius mirror to scale is 1.719 meter, 

 hence angular value of 1 division of scale = 1'. 



(1) Adjustment and determination of scale value, September 11, 1882, l h p. m. 



With plane of detorsion in the magnetic meridian the torsion circle read 54 42'. It was then 

 turned with the suspended weight 90, and read 324 42', in which position the fixed as well as the 

 movable mirrors were made to read 500 on the scale. The torsion weight was .then removed and 

 the magnet inserted and the torsion circle turned to read 248 35'. The movable mirror was next 

 brought to read 500, by means of the screw regulating the distance between the two suspension 

 threads. The angle 2=324 42' 248 35' = 70 07' was calculated to answer the desired value of 

 one division of the scale to represent a variation of the horizontal force of .001 cos. ft, expressed 

 in metric units (millimeter, milligramme, s). By inadvertence a mistake was made by the observers 

 in their calculation (in the value of if), so that the scale value neither for the horizontal nor for 

 the vertical force corresponds to the value proposed by the President of the Polar Commission. 

 This was not discovered by them until near the close of the observations, when they judged it best to 

 adhere to the old value. The magnetometers were thus given a sensitiveness fully double of what 

 was intended they should have. The consequence was that many of the largest disturbances in 

 the horizontal and vertical components failed to be registered, the deflections falling beyond the 

 range of the instruments. 



We have the scale value fc in parts of the horizontal force=cot. z times 1'=. 00007190, and 

 multiplying by H, or 1.939, the scale value becomes .0001394 English units. 



(2) September 18, 1882, 2 b a. m. to 3 h 15"' a. m., Gottingen time, readjusted bifilar instrument. 

 Plane of detorsion read 60 41' ; turned torsion circle to 330 41', and movable mirror made to 



read 50; magnet inserted and torsion circle turned to 254 34'; movable mirror brought to i-ead 50 

 by means of the adjusting screw. The angle z equals 76 07' ; hence fc, or the scale value, remains 

 as above. The apparent change in the plane of the detorsion of 5 59' is due to shifting of the 

 instrument. 



(3) November G, 1882, 10 h p. m., to November 7, 2 h 31 m a. m. Gottiugen time; readjusted in- 

 strument. 



With plane of detorsion in meridian torsion circle reads 52 46'; adjusted movable mirror to 

 50, when torsion circle reads 322 46'; suspended magnet and made torsion circle read 247 12'; 

 brought movable mirror to 50 by means of adjusting screw, z =75 34' ; hence Jc =. 00007487 parts 

 of the horizontal force, and multiplying by H the scale value becomes . 00014-52 English units. 



(4) February 27, 1883, 3 h 05'" a. in. to 6 h 55'" a. m. Gottiugeu time, readjusted instrument. 

 Plane of detorsion in magnetic meridian, torsion circle reads 52 35'; movable and fixed 



mirrors adjusted to 50, with torsion circle 322 35' ; suspended magnet and turned circle to 247 

 14' and brought movable mirror again to 50 by means of the adjusting screw, z = 75 21' ; hence 

 *=.00007G04 parts of the horizontal force, and the scale value .0()(>1474 English units. 



" Thus with !] Ki-y West (H = 0.74) limit of 2'.6 tho Ujjlaamio limit would he ;)' about. With the Philadelphia 

 (H = 4. 17) limit of 3.6 the Uglaamie limit would be*', about. With the Toronto m = 3.53) limit of 5.0 the V 

 limit would be <)', about. 



