Should the earth’s temperature increase at the rate of 1° for every 50 feet, it is clear that at the depth of 200 miles the hardest substances must be in a state of fusion, and our globe must in that case either be encompassed by a stratum of melted lava at that depth, or it must be a ball of liquid fire 7600 miles in diameter, enclosed in a thin coating of solid matter; for 200 miles are nothing when compared with the size of the earth. No doubt the form of the earth, as determined by the pendulum and arcs of the meridian, as well as by the motions of the moon, indicates original fluidity and subsequent consolidation and reduction of temperature by radiation; but whether the law of increasing temperature is uniform at still greater depths than those already attained by man, it is impossible to say. At all events, internal fluidity is not inconsistent with the present state of the earth’s surface, since earthy matter is as bad a conductor of heat as lava, which often retains its heat at a very little depth for years after its surface is cool. Whatever the radiation of the earth might have been in former times, certain it is that it goes on very slowly in our days; for M. Fourier has computed that the central heat is decreasing from radiation by only about the 1/30000th part of a degree in a century. If so, there can be no doubt that it will ultimately be dissipated; but as far as regards animal and vegetable life, it is of very little consequence whether the centre of our planet be liquid fire or ice, since its condition in either case could have no sensible effect on the climate at its surface. The internal fire does not even impart heat enough to melt the snow at the poles, though nearer to the centre than any other part of the globe. The immense extent of active volcanic fire is one of the causes of heat which must not be overlooked.