From an analysis of the available data, L. A. Bauer has con cluded that, over the period 1843-1923, the average intensity of magnetization of the earth has been decreasing at the rate of about one part in 1,5oo per year. A digest of the available data for the declination is given in Table III., for the period 1540-1920), and Table illustrates the variation of intensity, declination and inclination over a period of 20 years (1905-25).
A method which has been used for illustrating secular change is as follows : Radii are drawn from the centre of a sphere parallel to the direction of the freely dipping needle, and are produced to intersect the tangent plane drawn at a point which answers to the mean position of the needle during the epoch under consideration. The curve formed by the points of intersection shows the char acter of the secular change. Fig ure represents such a curve for London. The direction of description of the curve is clock wise, although this conclusion does not hold, apparently, for all places on the earth's surface. The inference suggested is that the curve will eventually close, indi cating a periodic phenomenon with a period of about 48o years.
All the magnetic elements at any ordinary station show a regu lar variation throughout the day. To separate this from the ir regular changes, means of the hourly readings must be formed, making use of a number of days. The amplitude of the diurnal change usually varies considerably with the season of the year, and to an extent depending upon the geographical location. The curves in fig. 4 constructed by D. L. Hazard') illustrate the general nature of the phenomena. They are based on days selected because of their freedom from magnetic storms, and represent the average of a large number of days.
A convenient method of exhibiting the diurnal variations of terrestrial magnetism is through the agency of vector diagrams, which give rather more complete information than is given by such diagrams as fig. 4. In such a vector diagram, a vector rep resenting in magnitude arid direction the earth's total intensity is drawn from the point corresponding to the place of observation. The motion of the end of this vector then contains in three dimensions the story of the variations of all the elements. In particular, the projection of this vector on a horizontal plane contains the story of the variation in the horizontal intensity.
By attaching to such a curve figures representing the time of day, the time variations of the element may be exhibited on the diagram. The sort of information representable in such a manner is indicated by the curves in fig. 4, which are drawn with par ticular reference to the influence of sunspot activity on the diurnal variation.
The more or less regular variations presented by the diurnal variation curves of fig. 4 are by no means characteristic of the
individual days. On certain days the elements vary every where over the earth's surface, and to an extent large compared with the normal diurnal variation. The larger disturbances of this type are known as magnetic storms. Days particularly free from such disturbances are termed quiet days. As a rule the condi tions lie somewhere between the two extremes. The more impor tant disturbances occur almost simultaneously all over the earth and for this reason it is possible to speak of the magnetic char acter of a day regardless of the location, the day being measured in terms of Greenwich time. It is customary for magnetic ob servatories to assign "Character figures" o, 1, or 2 to each suc cessive day, o denoting a quiet day, 2 a disturbed day, and i a day of intermediate type.