warmed by radium by an amount equal to the consumption of 9 million tons of coal a day. This rate of evolution of heat is sufficient to explain the whole of the temperature gradient in the earth's crust, assuming that it receives no heat whatever from the internal nucleus. If the centre were hot the added heat would render the earth's surface uninhabitable.
In the second place the temperature gradients, while dependent on the radium content of the rocks in the neighbourhood and on the chemical action going on in them, show generally that the older the rocks—that is, those that have been once deeply buried and have been nearer the earth's centre—the slower are the gradients, as on the Witwatersrand, where the increment is 1° F. for 254 ft.; but the more recent the rocks the more rapid does the increment become, so that we must assume that the increment as one goes deeper becomes less and less, until at a certain stage no further rise takes place, and from here downwards to the cold centre the temperature decreases. Where the zone is in which the temperature increase stops is a matter of speculation; it is probably not more than 20,000 or 25,000 ft. below the surface. It is this fact, quite inexplicable on the theory of a hot earth centre, that nearly stopped the boring of the Simplon tunnel through the Alps, because the granite encountered belonged to the younger rocks of the earth's crust and the temperature gradient was rapid; whereas in the Witwatersrand mines, where the temperature gradient is small, owing to the rocks being extremely old, the temperature at the bottom of the even deep levels is moderate enough to allow working.
The Size and Shape of the Earth.—The size of the earth was first measured by an African, Eratosthenes, a priest
