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  Probably the rings had to arrive at a certain stage of density before contracting circumferentially 139 Possible average temperature of the sun at the present day.     Central heat probably very much greater 140 Churning of matter going on in the interior of the sun, caused by unequal     rotation between the equator and the poles 140
CHAPTER VIII.
  Inquiry into the Interior Construction of the Earth.     What is really known of the exterior or surface 142 What is known of the interior 143 Little to be learned from Geology, which reaches very few miles down 144 Various notions of the interior 145 What is learnt from earthquake and volcanoes. Igno-aqueous fusion, liquid magma. 146 Generally believed that the earth consists of solid matter to the centre.     Mean density. Surface density 147 More detailed estimate of densities near the surface 148 Causes of increased surface density after the crust was formed 148 Calculations of densities for 9 miles deep, and from there to the centre forming Table IV. 150 Reflections on the results of the calculations 151 Notion that the centre is composed of the heaviest metals.   "Sorting-out" theory absurd 151 Considerations as to how solid matter got to the centre 152 Gravitation might carry it there, but attraction could not 153 How the earth could be made out of cosmic matter, meteorites or meteors 154
CHAPTER IX.
  Inquiry into the Interior Construction of the Earth—continued 165 The earth gasiform at one period. Density including the moon may have been 1/10,000th     that of air. Must have been a hollow body. Proofs given 166 Division of the mass of the earth alone into two parts 169 Division of the two masses at 817 miles from surface 171 Reasons why the earth cannot be solid to the centre 172 Gasiform matter condensing in a cone leaves apex empty 172 Proportions of the matter in a cone 173 Calculations of the densities of the outer half of the hollow     shell of the earth. Remarks upon the condensation 174 Calculations of inner half of the hollow shell 175 Remarks upon position of inner surface of the shell 177 Calculations of the same 179
CHAPTER X.
  Inquiry into the interior construction of the Earth—continued 184 Density of 8·8 times that of water still too high for the     possible compression of the component matter of the earth as known to us 185 Reasons for this conclusion drawn from crushing strains of materials 186 A limit to density shown thereby 187 The greatest density need not exceed 6·24 of water 188 Gases shut up in the hollow centre. Their weight must so     far diminish the conceded maximum of 6·24 189 Density of inner half of earth at 3000 miles diameter.     Greatest density may be less than 5·833 of water 190 Supposed pressure of inclosed gases very moderate 191 Meaning of heat limit to density. Temperature of interior     half of shell and inclosed gases must be equal 193 State of the hollow interior 194 Results of the whole inquiry 195
CHAPTER XI.
  The Earth. The idea entertained by some celebrated men, and others 197 Difficulties of forming a sphere out of a lens-shaped nebula 199 Various studies of the earth's interior made for specialy purposes. Difficulty some   people find in conceiving how the average density of little over 5·66 can be     possible, the earth being a hollow sphere 200 What is gained by its being a hollow shell 201 Geological theories of the interior discussed.     Volcanoes and earthquakes in relation to the interior 202 Liquid matter on the interior surface of the shell, and gases in the hollow,     better means for eruptions than magma layers 206 Focal depths of earthquakes within reach of water, but not of lavas 207 Minute vesicles in granite filled with gases, oxygen and hydrogen, but not water 209 The Moon. A small edition of the earth 211 Rotation stopped. Convulsions and cataclysms caused thereby. Air, water,     vapour driven off thereby to far-off hemisphere. Liquid matter in hollow     interior would gravitate to the inside of the nearest hemisphere 212 Form and dimensions during rotation. Altered form after it stopped 213 Agreeing very closely with Hansen's "curious theory"