TERRESTRIAL FIELD. 587 



of a magnet sets movable about a vertical axis. This latter ob- 

 servation usually presents some difficulties, since the magnetic 

 axis of a magnet is not generally parallel to its axis of figure; the 

 error is corrected by turning the magnet upside down, observing in 

 each position the direction of a line of sight ; the mean of the two 

 azimuths is the line which passes through the magnetic axis. 



The line of sight is formed either by the ends of a needle, cut in 

 the shape of an acute lozenge, or by two cross wires at the ends of a 

 bar, as in Gambey's compass ; or by two lines traced on the terminal 

 faces of the magnet, and which are viewed with the microscope. We 

 may also use hollow bars, converted into collimators by an object- 

 glass let into one end, and a divided scale or a cross wire at the 

 other. 



1160. A declination compass, or declinometer, is a true theo- 

 dolite, provided with accessory pieces for magnetic observations. 



In the older instruments, such as those of Gambey, and Gauss' 

 declinometer, the magnet was usually of great length, which has 

 many inconveniences, the principal of which is the slowness of oscil- 

 lation. In fact, for the same mean intensity of magnetisation, and 

 for bars of the same shape, the magnetic moment is proportional 

 to the volume, and the moment of inertia to the product of the 

 volume by the square of the length ; the time of oscillation is then 

 proportional to the length. It is impossible to stop the bar in its 

 position of equilibrium, and the slowness of the oscillations renders 

 the observations very long. 



We shall describe, as an example, the last model of a compass 

 constructed by MM. Brunner (Fig. 240). The magnet is a square 

 prism, furnished with two pins in the centre ; at each end is a silver 

 disc, on which is traced a division. This magnet is placed in a 

 stirrup, suspended by a cocoon fibre, which passes over a pulley I 

 at the top. The magnet moves in a metal box, terminated by two 

 glass plates with parallel faces, and mounted on the movable part 

 of the theodolite. The horizontal axis supports a microscope M, 

 which can pass under the box, and read the ends of the mag- 

 net; a cross wire, with three equidistant wires, serves for the 

 sighting. 



A small plane, which can be raised or lowered by a screw outside, 

 makes it possible to stop the oscillations. 



Finally, the wheel is mounted on a small movable divided circle, 

 and by lateral screws the wire of suspension can be centred ; so that, 

 for a given position of the magnet, the microscope points alternately 

 on the middle lines of the division. 



