The direction of the Earth's magnetic field at London, 1570-1975

Abstract

The observations of magnetic declination and dip made in London over the past 400 years comprise one of the longest and most complete series of such data for anywhere in the world. They give an indication of secular change on a time scale intermediate between that of recent observatory measurements and of archaeomagnetic measurements. We have attempted to catalogue all direct measurements of declination and dip, paying particular attention to completeness in the earlier years, and tracing the data to their original source wherever possible. Sufficient details of the observations are given for an assessment of their reliability and accuracy. The data are then corrected to a single site, fitted with a continuous smooth curve, and discussed.