Mr. O.Fisher — Changes of Latitude on the Earth's Surface. 293 



been investigating the above question raised by me. We may, there- 

 ore, hope to receive a decisive answer to it. 



It does not, however, appear necessary that geologists should be 

 concerned to inquire whether the earth is wholly fluid within. If 

 there be granted a layer of fluid matter of a thickness even very 

 small compared to the radius of the earth, that will suffice its. 

 May not the argument for rigidity drawn from the tides, after it 

 has received that definite weight which proper observations (yet 

 to be made) are expected to give it, be satisfied with a rigid 

 nucleus of radius nearly approaching to that of the entire globe ? 

 Such tides as would be formed within the liquid substratum of 

 the crust would not be of the nature of the tides, contemplated 

 by Sir W. Thomson as affecting the entire spheroid, but more 

 nearly analogous to the ocean tides ; since they would involve a 

 horizontal transference of fluid backwards and forwards, and might 

 be expected to be of small amplitude, owing to the viscosity of the 

 substance and its confinement beneath the crust. However, even 

 the modest requirement of a fluid substratum is denied to us. 

 " But now thrice to slay the slain. Suppose the earth this moment 

 to be a thin crust of rock or metal resting on liquid matter. Its 

 equilibrium would be unstable ! And what of the upheavals and 

 subsidences ? They would be strikingly analogous to those of a 

 ship that had been rammed ; one portion of crust up and another 

 down, and then all down. I may say with almost perfect certainty 

 that whatever may be the relative densities of rock, solid or melted, 

 at or about the temperature of liquefaction, it is, I think, quite cer- 

 tain that cold and solid rock is denser than hot melted rock, and no 

 possible degree of rigidity in the crust could prevent it from break- 

 ing in pieces and sinking wholly below the liquid lava." 



If we now turn to Mr. Hopkins' Kesearches in Physical Geology,^ 

 we find that he had gone into this question: somewhat fully, and 

 held an exactly opposite opinion from the above. He considered 

 that, so long as the matter of the earth retained a sufficiently high 

 state of fluidity to admit of the circulation of convection currents, 

 no crust could form : but that when, by those means, the temperature 

 had been so far reduced that convection became arrested, immediately 

 a crust would be formed. " Since the heat increases with the dis- 

 tance from the surface, while the mass is cooling by circulation, the 

 tendency to solidification, so far as it depends on this cause, will be 

 greatest at the surface, and least at the centre. But on the other 

 hand, the pressure is least at the surface, and greatest at the centre ; 

 and consequently the tendency to solidify as depending on this 

 cause will be greatest at the centre, and least at the surface." For 

 want of experimental evidence, " the only conclusion at which we 

 can arrive is this, that if the augmentation of temperature with that 

 of the depth be so rapid that its effect in resisting the tendency to 

 solidify be greater than that of the inci'ease of pressure to promote 

 it, there will be the greatest tendency to become imperfectly fluid, 



' Trans. Eoy. Soc. part ii. 1839, quoted in his Eeport to the British Association, 

 1848. 



