MR. DAVISON has laid geologists under many obligations to him for his mathematical investigations of vexed or obscure questions. His suggestion in the Geological Magazine, that the glacial period would probably have left a long-enduring mark upon the iso-geotherms, seemed to me, as I dare say it did to other students of glacial geology, a promising one; and though a comparison, which I made of the gradients in thirty-seven cases within the glaciated area of Britain with sixteen in the unglaciated portion, failed to reveal any significant difference, still I have been disposed to ascribe the failure rather to the imperfection of the data than to any fault in the method. When, however, Mr. Davison (NATURE, June 11, p. 137) extends the application of his formula to a comparison of two hemispheres, the insufficiency of the data is such as to entirely vitiate any results.