The declination of the needle varies from the meridian of Greenwich at different times. If we travel to the west the variation increases westerly, and is greatest in the Atlantic. It then decreases; the needle points due north in North America. Going still forward the variation becomes easterly, increases, and decreases to nothing in Eastern Russia. Thence the variations are westerly. Columbus discovered the variation of the compass needle in September 1493. In places where the needle is due north and south the lines drawn through them are termed lines of no variation. The variation, however, is not always the same in the same place. In the year 1580, in London, for instance, the variation was 11° 11´ E. A little more than one hundred years later London was on the line of no variation, and now the tendency is westerly. On the other hand, there are places where there is no deviation, and Sir John Herschell says that West India property has been saved from litigation in consequence of the invariability of magnetic declination there, for all surveys were made by the compass. Lines of equal variation are called isogonial; those of equal dip or inclination, isoclinical; and those of equal intensity, isodynamical. As we have said, the magnetic elements are not always the same, and the variations of the compass are daily and annually observed with certain instruments. What are termed secular variations take place at long intervals, as the following table will show:— In 1576 the angle of declination in England was 11° 15´ East. 1622 ” ” 6° 12´ ” 1660 ” ” 0° ” ” 1730 ” ” 13° West. 1760 ” ” 19° 30´ ” 1800 ” ” 24° 36´ ” 1818 ” ” 24° 41´ ” 1850 ” ” 22° 29´ ” 1870 ” ” 19° 55´ ” 1873 ” ” 19° 58´ ” 1879 ” ” 19° 7´ ” In the year 1818 therefore the maximum declination was reached in London. In Paris the maximum was arrived at in 1814, and was 22° 34´. The rate of decrease is about 8´ a year, but varies in different periods, as may be seen. The discovery of the fact that an annual variation took place in the angle of declination, is attributable to Cassini, and the diurnal variation was discovered by Graham in 1722. From 8 o’clock in the morning, when the needle is pointing a little to the east of its “mean position,” it turns towards the west until 1 p.m. It then returns towards the east again, and passing westerly again between midnight and three o’clock a.m., settles down till eight a.m., when it begins afresh. This variation does not apply to all places.