An ordinary pocket compass, orany instrument containing a magnetic needle, will give the direction of the magnetic north at the time and place where the compass is used. The direction of the magnetic north with regard to any given line of the survey can be ascertained by standing on the line and looking across the face of the compass, but it must be remembered that the direction of the magnetic north or, in other words, the magnetic meridian, is not constant. It is the direction of the mean resultant of the magnetic forces in the earth, and the virtual centre of the forces travels round the geographical north pole, so that in the neighbourhood of London the needle has a range of 30° east and west of the true north. The position of the needle was at the beginning of 1900 something less than 16 1/2 west of the true north, and this distance is being reduced at the rate of about 7' per annum.