New Theory on the Earth's Interior

Sky and Telescope of February contains a brief account ofthe theory of R. B. Borcherds which he advocated m his address as retiring chairman of the Cape Centre of the Astronomical Society of

South Africa. This theory suggests that the characteristics of tne earth's interior can be described on the basis of variations in condition rather than composition. Assuming a

temperature-gradient of one degree for every 90 ft. of depth, he finds that the observed discontinuities in the transmission of earthquake waves at depths of 7½ , 23 and 38 miles can be

attributed to changes of state due to high temperature and pressure. He describes a transitional region between 750 and 1,800 miles within which depths the temperature and pressure are

estimated to increase respectively from 44,000° to 105,000° C., and from 5,150,000 Ib. to 12,300,000 Ib. per square inch. The pressure at the centre of the earth is calculated at 50 million

pounds per square inch, and in such circumstances anounce of hydrogen would be compressed into 0-001 to 0-003 of a cubic foot. If 100,000° is above the critical temperature for any of the

known elements, no pressure applied to these substances at this temperature would cause them to liquefy or solidify; hence he considers that the earth's centre is gaseous, although highly

compressed. The gas would be ionized and compounds would produce much higher densities than the known average of 5-52 for the earth. He thinks that iron would lose its magnetic properties

under the intense heat at the earth's centre, so the old iron-nickel theory of the earth's core, developed to explain the earth's magnetic properties, cannot be sustained, and in any event

is unnecessary if rotating bodies develop magnetism, as has been recently found for stars.

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