Some suppose the central parts of our globe to be _cavernous_ or _hollow_. The principal argument for this theory is the transmission of _sound_ and _motion_ through vast extents of country, in case of volcanoes and earthquakes. It is supposed this could not be done so perfectly and extensively, unless we suppose some _aëriform_, or _gaseous_ body within the earth, by means of which it might be transmitted: which would be to suppose it _cavernous_ or _hollow_. Dr. Halley supposes the earth is a hollow sphere, in which there is inclosed a central magnetic globe, and by the motions of this globe the variations of the magnetic needle are produced. Our own ingenious, but unfortunate countryman, _Symms_, supposed the earth to be hollow, and inhabited within, and its interior accessible to us. He argues, there is no necessity, for the purposes of gravitation, or for any other purposes, to suppose the earth solid to the centre: And it is inconsistent with the divine beneficence to suppose such an amount of matter as this globe would be, if solid, should have been created to afford so small a portion, _scarcely one-fourth_, fit for the actual habitation of man, for whom principally it was created. He, therefore, supported, that the interior of the earth was an immense cavern blessed with changes of season, succession of day and night, cold and heat, and inhabited by human beings, and other animals. He supposed the poles of the earth were hollow, and this hollow entrance gradually verged round towards the equator; and ships have, without knowing it, been within the verge, from whence they found no difficulty of returning. Others have supposed the central parts of our globe are solid. This is the common supposition, and is principally supported by these two arguments:--As the attraction of gravitation depends on the _quantity_ of matter, as well as the distance; unless we suppose the earth a _solid_ body it will not be able to exert a sufficient attractive influence on the moon to keep her in her orbit. Again: it is ascertained by actual experiment, that the _mean_ density of the earth is about _five_ times that of water: from which it is infered it is solid, and must increase in density from the surface to the centre, in order to give this high mean proportion over the bodies at its surface. The increasing density of the earth, from the surface to the centre is owing to _compression_ in part, and partly to the supposed fact, that the heavier substances are placed nearer the centre. Thus we find the different strata of rocks indicate the same. Granite is the heaviest and lowest rock _in situ_.