The _meridian line_ at any place is the intersection of the plane of the meridian with the plane of the horizon at that place, or, in other words, it is the line joining the north and south points. If we have the proper instruments, we can determine the meridian line astronomically at any place by one of the following methods:-- (1) If only an approximate position is required, the best means of determining it is by fixing the direction of the sun or a star when it has the greatest altitude. The instrument to be used for this purpose would be a small theodolite with both a vertical and horizontal circle, and provided also with tangent screws to give slow motion to each of the circles as required. By using stars of both high and low altitudes, a greater exactness can be obtained, but, after all, the method only gives a first approximation, as its weakness lies in the very slow change of altitude as the meridian is approached. (2) A much more accurate method is that of observing with an altitude and azimuth instrument the azimuth (_i.e._, its angular distance east or west of the north or south) of a star when at the same altitude east and west of the meridian. If the mean of the two readings given by the azimuth circle be taken, the resulting reading indicates the direction of the meridian. If we employ the sun in place of a star, its change of declination during the interval between the observations must be taken into account. (3) To find the meridian line by means of the pole star is a simple and accurate method, as a value can be obtained at _any_ time at night by a simple altitude, provided the time of observation is known.[8] If these means of directly determining the astronomical meridian line are not available, then we have to do it indirectly by using the magnetic meridian in the first instance. If we take a magnetic needle and balance it horizontally on a vertical pivot, its ends will be directed to two points on the horizon. By drawing a great circle through these two points and the zenith point of the place, we obtain the _magnetic meridian_. The _magnetic meridian line_ is the intersection of the plane of the magnetic meridian with the plane of the horizon. The angle between the astronomical and magnetic meridian lines is called the variation, E. or W. according as the needle points to the W. or E. of true--that is, astronomical--north at any particular place at any particular time. The variation may vary from place to place, and always varies from time to time. The bearing required has, in the first instance, to be determined by the instruments already referred to in relation to the magnetic meridian. Having made such an observation, the next thing we have to do is to determine the astronomical or true north, which is the only thing of value. [Illustration: MAGNETIC MAP OF THE BRITISH ISLES, SHOWING THE VARIATION AT DIFFERENT POINTS.]