It must not be supposed because the rate of increase of temperature is not uniform that the argument of a highly heated interior is weakened. On the contrary, it would be very surprising if the rate continued uniform; for it is evident that the conducting power of different materials in the earth's crust for heat must necessarily make a great difference in the rate at which heat should increase, as we go farther down into the earth. This is so important a matter that I will explain it at somewhat greater length. Let us suppose that instead of the highly heated interior of the earth, we consider the simple case of a hot stove, the doors or other openings into which are closed so that it is impossible to see the red hot coals inside. Now, suppose holes were bored in the sides of this stove not deep enough to reach the red hot mass within, and that tightly fitting rods or plugs all of the same length and thickness, but of different kinds of materials such as wood, earthenware, glass, iron, copper, silver, and gold, etc., were so placed in the holes as to tightly fit them. Now, under these circumstances the end of all the plugs would be at the same distance from the heated inside. They would not, however, by any means show the same temperatures, the metallic rods would be too hot to touch, while the end of the piece of wood would hardly be hot enough to burn the hand when held against it. The piece of glass and earthenware though less cool would be much less hot than the different rods of metals. Their temperatures would be necessarily affected by their conducting power for heat. The wood, the glass, and the earthenware being poorer conductors than the metals would show much lower temperatures. Now, the same thing is true with the different materials that constitute the rocks of the earth's crust. Some of these are much better conductors of heat than others, so that the rate of increase of temperature with descent below the surface must necessarily vary with the kind of materials that form the crust of different parts of the earth. You may, therefore, safely conclude that the entire interior of the earth is in a highly heated condition, and that the source of this heat is to be traced to the heat the earth originally possessed when, in accordance to the nebular hypothesis of LaPlace, it was separated from the sun which gave birth to it, that the present crust of the earth has been formed on the outside by the loss of a portion of this heat.