to the Antarctic Regions. 



229 



being thus found, the true declination changes are deduced from the 

 apparent, by multiplying them by the coefficient 



In order to olitain an exact result by the mode of experiment above 

 described, it is necessary that the actual changes of the declination 

 which may occur in the interval of the two readings, should be eli- 

 minated. The obvious method of accomplishing this, is to observe 

 the decHnation changes simultaneously with a second apparatus. If 

 such means, however, should not be at hand, the object may be at- 

 tained by making a series of readings with the vernier of the torsion 

 circle alternately in two fixed positions (for example + 90° and 

 — 90°) ; the mean result will be independent of the declination 

 changes, provided the progress of these changes has been gradual 

 in the interval of the experiment. 



For the purpose of determining the absolute intensity of the hori- 

 zontal component of the earth's magnetic force, the declination in- 

 strument is provided with a defecting bar, and a beam compass to be 

 used in measuring its distance from the suspended magnet. The 

 mode of observation has been so fully explained by Gauss, in his 

 valuable memoir entitled " Intensitas vis terrestris ad mensuram ab- 

 solutam revocata," and in the first volume of the " Resultate*," that 

 it is unnecessary to enter here into any details. 



The following table contains the interval of the sliders of the col- 

 limators, corresponding to focal adjustment ; and also the arc values 

 of one division of the scale in each instrument, expressed in deci- 

 mals of a minute. 



Table I. 



Horizontal Force Magnetometer. 



The instrument employed in determining the horizontal compo- 

 nent of the earth's magnetic force is similar, in principle, to the 

 " bifilar magnetometer" of Gauss. It is a magnet bar, suspended by 

 two equidistant wires, or (more accurately) by two portions of the 

 same wire, the distance of whose bearing points is the same above 

 and below ; by the rotation of the upper extremities of the wire 



* Translated in Taylor's Scientific Meinoiis, vol. ii, part v. 



