The encyclopaedia of ignorance: Ronald Duncan and Miranda Weston-Smith (Editors). Pergamon Press,Oxford, 443 pp., ISBN 0-08 022426-1, £ 7.50 (flexicover).

Analysis of geomagnetic secular variation data from world observatories demonstrates the occurrence of a secular variation impulse in the late 1960's. When a simple mantle conductivity model is used in conjunction with the impulse data, it can be shown that the average conductivity of the lower mantle does not exceed 150 ω^?1 m^?1. This is orders of magnitude smaller than recently published values.