When making a measurement the azimuth circle is first set horizontal, a striding level placed on the trunnions which carry the ring being used to indicate when the adjustment is complete. The striding level is then placed on the axle which carries the coil, and when the bubble is at the centre of the scale the microscopes are adjusted to the zeros of the vertical circle. A box containing a long compass needle and having two feet with inverted V’s is placed to rest on the axle of the coil, and the instrument is turned in azimuth till the compass needle points to a lubber line on the box. By this means the axis of the coil is brought into the magnetic meridian. The commutator being connected to a sensitive galvanometer, the coil is rotated, and the ring adjusted till the galvanometer is undeflected. The reading on the vertical circle then gives the dip. By a system of reversals slight faults in the adjustment of the instrument can be eliminated as in the case of the dip circle. With such an instrument it is claimed that readings of dip can be made accurate to ±0.1 minutes of arc.
The form of Wild inductor for use in a fixed observatory differs from the above in that the coil consists of a drum-wound armature, but without iron, of which the length is about three times the diameter. This armature has its axle mounted in a frame attached to the sloping side of a stone pillar, so that the axis of rotation is approximately parallel to the lines of force of the earth’s field. By means of two micrometer screws the inclination of the axis to the magnetic meridian and to the horizontal can be adjusted. The armature is fitted with a commutator and a system of gear wheels by means of which it can be rapidly rotated. The upper end of the axle carries a plane mirror, the normal to which is adjusted parallel to the axis of rotation of the armature. A theodolite is placed on the top of the pillar and the telescope is turned so that the image of the cross-wires, seen by reflection in the mirror, coincides with the wires themselves. In this way the axis of the theodolite telescope is placed parallel to the axis of the armature, and hence the dip can be read off on the altitude circle of the theodolite.
Authorities.—In addition to the references already given the following papers may be consulted: (1) Admiralty Manual of Scientific Inquiry, which contains directions for making observations with a dip circle; (2) Stewart and Gee, Elementary Practical Physics, which contains a full description of the dip circle and instructions for making a set of observations; (3) L. A. Bauer, Terrestrial Magnetism (1901), 6, p. 31, a memoir which contains the results of a comparison of the values for the dip obtained with a number of different circles; (4) E. Leyst, Repertorium für Meteorologie der kaiserl. Akad. der Wiss. (St Petersburg, 1887), 10, No. 5, containing a discussion of the errors of dip circles; (5) H. Wild, Bull. de l’Acad. Imp. des Sci. de St Pétersbourg (March 1895), a paper which considers the accuracy obtainable with the earth inductor.
(W. Wn.)
[1] Repertorium für Meteorologie der kaiserl. Akad. der Wissensch. (St Petersburg, 1892), 16, No. 2, or Meteorolog. Zeits. (1895), 12, p. 41.
INCLOSURE, or Enclosure, in law, the fencing in of waste or common lands by the lord of the manor for the purpose of cultivation. For the history of the inclosure of such lands, and the legislation, dating from 1235, which deals with it, see COMMONS.