1. Relation to Magnetic Survey.—After being engaged for sometime, in conjunction with Professor Thorpe, upon the Magnetic Survey of the United Kingdom, Professor A. W. Rücker, F.R.S., put forward his theory of Terrestrial Magnetism.
In connection with this theory, it was desirable to determine the permeabilities of various rocks, both at ordinary and at high temperatures.
2. The investigation which forms the subject of the present paper is an instalment in this direction. Professor Rücker proposed this undertaking and indicated the plan to be followed. It was then carried out under his supervision at the Royal College of Science, London.