1. magnetic elements The qualities of the terrestrial magnetism at any place as expressed in its action upon the magnetic needle. Threedata are involved.
Table of Declination or Variation at Paris. Year. Declination. 1580 11º 30' E. 1663 0° 1700 8° 10' W. 1780 19º 55' W. 1785 22º 00' W. 1805 22º 5' W. 1814 22º 34' W. 1825 22° 22' W. 1830 22º 12' W. 1835 22º 4' W. 1850 20º 30' W. 1855 19º 57' W. 1860 19º 32' W. 1865 18º 44' W. 1875 17º 21' W. 1878 17º 00' W. [Transcriber's note The value for 2008 is about 0° 48' W, changing by 0° 7' E/year.]
On scrutinizing these figures it will be seen that there is part of a cycle represented and that the declination is slowly returning to the zeropoint after having reached its maximumwesternvariation in 1814. Upwards of 300 years would be required for its completion on the basis of what is known. In other places, notably the coast of Newfoundland, the Gulf of the St. Lawrence and the rest of the NorthAmericanseaboard and in the British Channel, the secular variations are muchmorerapid in progress. (b) Annual variations--These were first discovered in 1780 by Cassini. They represent a cycle of annualchange of small extent, from 15' to 18' only. In Paris and London the annualvariation is greatest about the vernal equinox, or March 21st, and diminishes for the next three months, and slowly increases again during the ninefollowing months. It varies during different epochs. (c) Diurnal variations were discovered in 1722 by Graham. A long needle has to be employed, or the reflection of a ray of light, as in the reflecting galvanometer, has to be used to observe them. In England the northpole of the magnetic needle moves every day from east to west from sunrise until 1 or 2 P. M.; it then tends towards the east and recovers its originalposition by 10 P. M. During the night the needle is almost stationary. As regards range the meanamplitude of diurnal variations at Paris is from April to September 13' to 15'; for the other months from 8' to 10'. On some days it amounts to 25' and sometimes is no more than 5'. The amplitude of diurnal variations decreases from the poles to the equator. Irregular variations accompany earthquakes, the aurora borealis and volcanic eruptions. In Polar regions the auroral variations may be very great; even at 40° latitude they may be 1° or 2°. Simultaneous irregularities sometimes extend overlarge areas. Such are attributed to magnetic storms. II. The Inclination is the angle which the magnetic needle makes with the horiz
Definitions of magnetic elements
