REFRIGERATION OF THE EARTH. That the earth is radiating more heat into space than it receives is evidenced by the fact that the temperature rises as the crust of the earth is penetrated. The flow of heat is always from a higher to a lower temperature, and consequently there is a constant conduction of beat from the interior to the surface of the earth. and a loss into space. The rate of flow of heat is dependent upon the conductivity of the material and the rate of rise in temperature. A substance is said to have the unit of heat conductivity when it will conduct the unit of heat the unit of distance in the unit of time through the unit cross-sec tion, the temperature rising one degree for the unit length. In the present system unit con ductivity corresponds to the transfer of enough heat to raise one pound of water one degree Fahr enheit in one second through a bar of one square foot eross-section when the temperature falls one degree for one foot in the direction of flow. De terminations of the increase in temperature as the earth's crust is penetrated show that there is a marked variation from one locality to another, this variation depending upon the character of the rocks penetrated, the amount of disturbance they have undergone, and other local conditions. In the well at Wheeling, W. Va.. which has a depth of 4,500 feet. the rise in temperature has been found to the 1° F. for each 75 feet of de scent: in the well at Sperenberg, Prussia (depth 4,170 feet), the increase is 1° for 59 feet; and in the Schladabach well, near Leipzig (depth 5,740 feet), the increase is 1° for 65 feet. At other
localities the rate varies from 15 to 200 feet for 1° F.
Various causes have been suggested for this interior heat of the earth, but considering all the related astronomical and physical considerations it is most probable that it is a remnant of the original nebular heat which has not yet been dis sipated. and which has been prolonged by the re sults of the condensation from the nebulous state. Under the phrase 'age of the earth' is usually understood the time since the earth became prac tically solid and the present method of cooling by conduction commenced. From data as to the conductivity of the rock materials and the rise in temperature it is possible to make certain estimates as to the age of the earth, and the latest and perhaps best is that by King, who, after an exhaustive discussion, concludes that it is about twenty-four million years and that the temperature at the time when convection ceased was about 2000° C. (3600° F.). The amount of heat reaching the earth's surface from the in terior in a year is sufficient to melt a layer of ice about one and a quarter inches thick, that is about one-thousandth of the amount reaching the earth from the sun. Consult article by ling, in .1 inerican Journal of Science (New Haven, 1893).