The celebrated Dr. Halley, in the year 1692, in his attempt to account for the change of the variation of the magnetic needle, advanced a novel hypothesis, as respects the internal structure of the earth. He supposes that there is an interior globe, separated from the external sphere by a fluid medium; or that there may be several internal spheres, separated from each other by atmospheres, and that the concave arches may in several places shine with a substance similar to that which invests the body of the sun, producing light and heat for the accommodation of those internal regions which he alleges may possibly be inhabited by animate beings.[45] However, he suggests no idea of Polar Openings, nor of any communication from the outer surface to those interior regions; consequently their existence must have remained forever a matter of mere conjecture. We find that Dr. Halley, in the wisdom of his philosophy, believed those internal regions to be lighted, though situated many thousand fathoms beneath the surface, and without any aperture to communicate light from without. Why not, then, believe that the interior of the spheres, according to Symmes's theory, may be lighted, when he lays down such vast openings at either pole for that purpose? Euler was also an advocate for the theory of Dr. Halley. He believed, with him, that the earth is hollow, with a ball, or nucleus, included in the centre; he, however, differed from Halley as to the nature of the nucleus. Halley believed it to be constituted of the same materials of the exterior crust of the earth. Euler believed it to be a luminous body formed of materials similar to the sun, and adapted to the purpose of illuminating and warming the interior surface of the crust, which he supposed might be inhabited equally with the exterior surface. He fancied that this luminous ball had no rotary motion, and that the outer shell revolved around it. However, neither he nor Dr. Halley left any opening by which the internal regions could be explored; their existence was therefore left to rest on vague hypothesis.[46] These different theories, however extravagant they may appear to us, were believed and supported by those men, whom we must acknowledge were among the most learned of the age in which they lived; and among the mathematicians in Europe they have yet some warm supporters. Why not then give Symmes's theory of open poles, and _concentric spheres_, a serious investigation, the correctness of which is so much more probable, and the demonstration of its truth or falsehood so much more practicable? At all events a _voyage to the polar regions_, with an eye to the accomplishment of Symmes's purpose, might be productive of incalculable advantages to the cause of science in general. With respect to astronomy and geography, it would afford many new lights, and perhaps discover and establish many new principles, not thought of at this day.