Hypothesis of the Internal Fluidity of the Earth. 135 



matter inclosed were mutually adjusted to an equilibrium. 

 The diminished capacity due to a loss of 3000° of surface tem- 

 perature would be about 33 J* millions of cubic miles. This 

 then would, according to hypothesis, be the volume of the 

 whole igneous rocks ejected since the surface of our globe 

 was possessed of the temperature above assumed. 



7. The aggregate width of the fissures opened in the crust 

 being (as by Par. 5) 333J miles in the direction of a great 

 circle of the earth, may be represented by two wedge-shaped 

 ditches surrounding the globe, one being equivalent to the 

 sum of the fissures in longitude, and the other to the sum 

 of fissures in latitude. The length of these ditches may be 

 taken at 24,876 miles, their width at the surface (average 

 being half of 333J miles) 166,833 miles,f and their depth 

 25-56 miles. Hence, 24876 x 166-833 x a^U x 2 = 

 105,971,760J cubic miles, the quantity of igneous matter re- 

 quired to fill up the whole of these fissures. This would be 

 equal to two ridges engirdling the earth, each having a 

 width at base of 166*83 miles, and a height of 25*56 miles. 



8. Supposing the density of the earth to increase in de- 

 scending in an uniform ratio, from 2 at the surface to 20 at 

 the centre, then at a depth of 23*1796 miles (or probably 

 not far from the upper limit of the molten matter of the 

 interior), the specific gravity of the materials there existing 

 should be 2*1055. Or, taking the surface density at 2, and 

 at the centre 22, we have at the above depth the specific 

 gravity of 2*1172. Taking the means of these, the density 

 at 23-1796 miles will be 2*1113, and taking 23*2 miles as the 



I thickness of the crust, its specific gravity on an average will 

 be 2-0556. 



9. Assuming as before the crust to have lost, upon an 

 average of the whole, 1500° of heat, and the resulting con- 



* 197,000,000 (square miles of earth's surface) x *ff£* = 33,580,000 cubic 

 miles. 



t A deduction from the width of the fissures at the surface should be made 

 on account of the co-resulting shortening of the radius ; but as this would 

 amount only to about the 300th part of the whole, it is unnecessary to take 

 it into account in the calculation. 



X 35,324 cubic miles would be required to fill up the fissures opened by the 

 contraction resulting from the crust parting with one degree of heat, 



