equator, and that the dip increases with the latitude, the north pole of the needle dipping in the northern hemisphere, and the south pole in the southern hemisphere.

At two points on the earth's surface the dip is 90°, i.e. the dippingneedle is vertical, this indicating that the horizontal component is zero, so that the compass-needle, which indicates the direction of the horizontal component, will not set itself at these points in any definite direction. These points are often called the magnetic poles. The north magnetic pole lies in lat. 70° 5' N. and long. 96° 43′ W., while the south magnetic pole is at lat. 73° 30′ S. and long. 147° 30′ E.

The agonic line passes through the magnetic poles.

The lines of equal horizontal force are shown in Fig. 424, and it will be seen that the horizontal force is a maximum near the equator, and is zero at the magnetic poles.

The curves of equal total force are called isodynamic lines. The total force is not a maximum at the magnetic poles, but there exist in the northern hemisphere two points at which the total force is a maximum, while two similar points exist in the southern hemisphere. These points of maximum force are called magnetic foci. One of the northern foci is situated in North America at lat. 52° N., and long. 90° W. The other northern focus is at lat. 70° N. and long. 115° E., and is called the. Siberian focus. The two southern foci are situated much nearer together than are the northern ones, their positions being approximately lat. 65° S., long. 140° E., and lat. 50° S., long. 130° E.

The positions of the terrestrial lines for the whole globe are necessarily only roughly known, for there are very large tracts where few, if any at all, determinations of the magnetic elements have been made. In the case of some more or less restricted portions of the earth, notably Great Britain, the magnetic elements have been determined with great accuracy at a large number of places, and hence the terrestrial lines are known with some accuracy. In Fig. 425 the lines of equal declination, dip, and horizontal force are given as obtained in an extensive magnetic survey conducted by Professors Rücker and Thorpe. These lines are obtained by combining the results of the measurements made at a number of stations which are grouped together so as to eliminate the effects of any local abnormality in the value of the elements at any one point, and hence they give what may be considered as the normal distribution of the lines. The value of the elements at any given spot do not, however, in general agree exactly with the values as deduced from these curves. This difference is due to the fact that at the place considered there may exist slight abnormalities, owing to the presence of magnetic material in the neighbouring portions of the earth's crust. The extent to which these disturbing causes may affect the even trend of the terrestrial lines is well shown in Fig. 426, which gives the form of the true isogonal lines, that is, the lines passing through the places at