to the Antarctic Regions. 231 



an inch nearly. The micrometer head has been carefully adjusted 

 by the artist, so that the index is at zero, when the interval of the 

 wires is exactly half an inch. 



The collimator, in this instrument, is inclosed in a light tube 

 attached to the stirrup. The aperture of the lens is about yyths of 

 an inch, and its focal length about 8 inches. The divisions of the 

 scale are the same as in the collimator of the declination magneto- 

 meter ; the corresponding arc values have been ascertained for each 

 instrument by accurate experiment, and are given in Table II. 



The larger parts of this apparatus, — the box, the framework, and 

 the support, — are precisely similar to those of the declination mag- 

 netometer. In addition to the parts already described, the instrument 

 is furnished with a spare magnet ; a brass weight, required in deter- 

 mining the plane of detorsion of the wires relatively to the magnetic 

 meridian ; a thermometer, the bulb of which is within the box, for the 

 purpose of ascertaining the interior temperature ; and a copper ring 

 used in checking the vibrations. 



Adjustments. — The instrument being placed on its support, the 

 base is to be levelled, and the whole apparatus fixed. Having then 

 selected one of the small grooved wheels, and fixed it, temporarily, 

 with its axis horizontal, the wire is to be passed round it ; and the 

 free extremities of the wire being passed through the corresponding 

 holes in the suspension roller, placed beneath, weights are to be 

 attached, and the two portions of the wire allowed to assume their 

 natural position ; the extremities may then he fastened to the roller, 

 by introducing small wooden plugs in the holes. The parts are then 

 to be inverted, and put in their proper places ; the suspension appa- 

 ratus resting on the divided circle, and the wire hanging down the 

 tube. 



The collimator (its scale having been previously adjusted to focus*) 

 is to be screwed on to the stirrup, and the latter attached to the axis 

 of the grooved wheel by means of its Y s. The magnet is then in- 

 troduced into the stirrup, and levelled ; and the wires wound upon 

 the roller, until the collimator is at the desired height. 



These preparations being made, the adjustments are the following: 



1 . Determine experimentally the angle through which it is neces- 

 sary to turn the moveable arm of the torsion circle, in order to deflect 

 the magnet from the magnetic meridian to a position at right angles 

 to it, the two positions being merely estimated. The cosine of this 



angle is, approximately, the ratio of the magnetic force to the tor- 



■p 

 sion force, or the value of the fraction — -. The nearer this ratio is 



Gr 



to unity, the more delicate will be the instrument ; practically, ^g 

 will be found a convenient value. If, on making the foregoing ex- 

 periment, the ratio should be found to fall below, or to exceed the 

 proper limits, the torsion force must be altered by introducing a dif- 

 ferent wheel, and making the corresponding alteration in the interval 

 of the upper extremities of the wires. 

 , 2. The magnetic axis being brought, approximately, into the mag- 



* This adjustment has been already made by the artist. 



