Page:The fairy tales of science.djvu/333

Rh temperature is acquired at a still less depth. Were we to proceed down to the very centre of the earth, we should there find, supposing a regular rate of progression in the increase of temperature, a heat exceeding 3500 degrees of Wedgewood’s pyrometer, or something like 450,000 degrees Fahrenheit! The solid crust of the earth is generally supposed to be only from 60 to 100 miles thick; and it is probably even much less; that the thickness is very unequal is shown by the variation of temperature, which cannot be attributed solely to different degrees of conductibility in different parts. The process of cooling from the crust downwards is, of course, still going on, but, as has been demonstrated by Fourier, at a less rate than was formerly the case. According to the same authority, it will require 30,000 years to reduce the increase of temperature on descending into the interior of the earth from its present rate of one degree Fahrenheit for every 60 feet in descent, to one-half degree. Some geological chemists have calculated from the known laws of radiation of heat, that it would take 200,000,000 years to cool the earth to its centre!

Another point to consider is the density of the earth. The density of the crust lies between 2·7 and 2·9; but we know, from most careful and accurate pendulum experiments, that the average density of the bulk of the earth is about 5·5. It is quite evident, therefore, that the ponderable matter of the interior must be very much denser than that of