If the earth is a hollow sphere, it must have an internal as well as an external surface. But how are we to find out what is the distance between these two surfaces? Let us, to begin, take a look at the hollow part of the sphere. From the time of Arago it began to be supposed that there is a continual deposit of cosmic matter upon the earth going on, and since then it has been proved that there is a constant and enormous shower of meteors and meteorites falling upon it. But although this is the case on the exterior surface, it may be safely asserted that on the interior surface, where the supply of cosmic matter must have been limited from the beginning, there can be no continual deposit of such matter going on now; nor can there have been from, at least, the time when the earth changed from the form of vapour to a liquid state. We may, therefore, be sure that there is no undeposited _cosmic_ matter of any kind in the hollow of the sphere, and that, as far as it is concerned, there is an absolute vacuum. TABLE V.--CALCULATIONS OF THE VOLUMES AND DENSITIES OF THE OUTER HALF OF THE EARTH--TAKEN AS A HOLLOW SPHERE--AT THE DIAMETERS SPECIFIED, AND REDUCED TO THE DENSITY OF WATER. With mean diameter of 7918 miles. Diameter of half-volume at 6284·5 miles, and density there of 8·8 times that of water. -----+-------+---------------+-------+----------------+--------------+ Diam.|Densi- | Volumes | Avgs. | Volumes at | Observations.| in | ties. | in Cubic | of |Density of Water| | miles| | Miles. |Density| in Cubic Miles.| | -----+-------+---------------+-------+----------------+--------------+ | | | | |{ Half-volumes| | | | | |{ of the earth| 7918 | ... |129,961,924,377| ... | 735,584,493,738|{ actual and | | |---------------| |----------------|{ at density | | | | | |{ of water. | | | | | | | | | | | |{ Density at | | | | | |{ 7914 miles | | | | | |{ in diameter.| 7914 | 2·0000| 393,724,522 | 1·0000| 393,724,522|{ The 2 miles | | | | | |{ above being | | | | | |{ at density | | | | | |{ of water. | 7905 | 2·5000| 491,596,266 | 2·2500| 1,106,090,598| | | | | | | | 7900 | 3·0000| 883,309,189 | 2·7500| 2,429,097,520| |