to the Antarctic Regions. 235 



Vertical FoncE Magnetometer. 



The instrument used in determining the changes of the vertical 

 component of the magnetic force is a magnetic needle resting on 

 agate planes, by knife-edges, and brought to the horizontal position 

 by weights. From the changes of position of such a needle, the 

 changes of the vertical force may be inferred, when we know the 

 mean inclination at the place of observation, the azimuth of the plane 

 in which the needle moves, and the angle which the line connecting 

 the centre of gravity and centre of motion makes with the magnetic 

 axis. As, however, the determination of this latter constant would 

 involve the necessity of considerable additions to the apparatus, the 

 plan adopted has been to adjust the needle so that the angle in 

 question shall be nothing. The centre of gravity being thus brought 

 to some point of the magnetic axis, the changes of the vertical force 

 are connected with the changes of position of the needle by the 

 formula 



— — = cosa . cotan . 6 d ^ ; 

 h 



d ^ denoting the change of angle in parts of radius, a the azimuth of 

 the plane in which the needle moves, and 6 the inclination. 



Construction. — The magnetic needle is 12 inches in length. It 

 has a cross of wires at each extremity, attached by means of a small 

 ring of copper; the interval of the crosses being 13 inches. The 

 axis of the needle is formed at one part into a knife edge, and at the 

 opposite into a portion of a cylinder, having this edge for its axis, 

 the edge being adjusted to pass as nearly as possible through the 

 centre of gravity of the unloaded instrument. The weights by 

 which the other adjustments are effected are small brass screws 

 moving in fixed nuts, one on each arm ; the axis of one of the 

 screws being parallel to the magnetic axis of the needle, and that 

 of the other perpendicular to it. 



The agate planes upon which the needle rests are attached to a 

 solid support of copper, which is firmly fixed to a massive marble 

 base. In this support there is a provision for raising the needle off 

 the planes, the contrivance for effecting this object being similar to 

 that employed in the inclination instrument. The whole is covered 

 with an oblong box of mahogany, in one side of which are two small 

 glazed apertures, for the purpose of reading ; the opposite side of 

 the box is covered with plate glass. A thermometer, within the 

 box, shows the temperature of the interior air ; and a spirit level, 

 attached to the marble base, serves to indicate any change of level 

 which may occur in the instrument. 



The position of the needle at any instant is observed by means of 

 two micrometer microscopes, one opposite each end. These micro- 

 scopes are supported on short pillars of copper, attached to the 

 base of the instrument. They are so adjusted that one complete 

 revolution of the micrometer screw corresponds to 5 minutes of arc. 

 The micrometer head is divided into 50 parts ; and, consequently, 

 the arc corresponding to a single division is 0'' 1 . 



