"There are almost as many conjectures as to that as there are geologists," was his answer. "One estimates it thirty miles, because the internal heat, increasing at the rate of about one degree to each sixty to seventy feet depth, would be sufficient to fuse the most refractory substances at that distance beneath the surface. Another finds that the phenomena of precession and nutation require that the earth, if not entirely solid, must at least have a shell not less than eight hundred to a thousand miles in thickness. So there you are. You may take your choice."
"And if it should prove solid?" I asked.
"It will be all the same to us in the end, David," replied Perry. "At the best our fuel will suffice to carry us but three or four days, while our atmosphere cannot last to exceed three. Neither, then, is sufficient to bear us in the safety through eight thousand miles of rock to the antipodes."
"If the crust is of sufficient thickness we shall come to a final stop between six and seven hundred miles beneath the earth's sur-
