The Theory of Concentric Spheres, supported by arguments drawn from Terrestrial facts; such as the migration of animals to and from the arctic regions, and from refraction, and the variation of the compass, observed in high northern latitudes. CHAPTER VI. Facts tending to illustrate and prove the existence of a _mid-plane space_, situated between the concave and convex surfaces of the sphere. CHAPTER VII. Several objections, made to the Theory of Concentric Spheres, answered, particularly the one that it contravenes religious opinions; demonstrating that the earth, and the other orbs of the universe, are formed on the best possible plan for the maintenance and support of organic life. CHAPTER VIII. General observations on the Theory of Concentric Spheres, with a few suggestions to the Congress of the United States, to authorize and fit out an Expedition for the discovery of the Interior Regions; or, at least, to explore the northern parts of the continent of America. CHAPTER IX. A few brief suggestions, relative to the description, tonnage, and number of vessels, necessary to be equipped for a voyage of discovery to the interior regions of the earth; the number of men necessary to be employed on board, articles necessary for the outfit, and the probable expense attending the same; also, as to the route most proper to be pursued to accomplish the object of the expedition. CHAPTER X. A short Biographical sketch of Captain Symmes; with some observations on the treatment which he has met with in the advancement of his theory. THE THEORY OF =CONCENTRIC SPHERES=. CHAPTER I. _Containing an introductory glance at some of the different Theories and Opinions which have been advanced respecting the formation of the Earth, and the reception which those Theories met with from the world when first promulgated._ It often happens, that those who have been early taught to believe a certain set of principles and doctrines as true, whether in philosophy, religion, or politics, adhere to them with the utmost pertinacity during the remainder of their lives. Any new theory, or principle, is resisted with peculiar energy; and, however inconsistent or untrue their favorite systems may be, they are disposed to make principles and facts bend to them; and would sooner call in question the general and immutable laws of nature, than the correctness of their own opinions. Perhaps this pertinacious adherence to prevalent and received opinions has retarded the progress of philosophic improvement more than the want of bold, original, and enquiring genius.