Physical Structure of the Earth. 235 



that the recognized laws of matter apply to the internal con- 

 dition of the Earth. The shape of the nucleus and the figures 

 of its strata of equal density follow from physical and me- 

 chanical laws, just as the forms of the isothermal surfaces 

 within the spheroid follow from the known laws of conduction 

 of heat. Some of the mechanical reasonings regarding the 

 strata of the nucleus and the structure of the solid shell can 

 he presented without employing mathematical symbols, and 

 in what follows I have as far as possible avoided the use of 

 such symbols. 



This course, moreover, possesses the advantage of making 

 many parts of the reasonings more clear to geologists and 

 observers of the stratigraphical features of the Earth, who are 

 in reality the ultimate judges of the matter, and not mathe- 

 maticians. The necessity under which the latter are con- 

 strained when dealing with problems, of throwing the pre- 

 liminary propositions into simple well-defined shapes, admitting 

 of definite deductions, obliges them to overlook the most 

 essential conditions of the very questions at issue, and they 

 thus arrive at results which may be precise, but which are 

 totally inconclusive with reference to the Earth's structure. 



The Mechanical and Physical Properties of the Matter 

 composing the JEarth. 



(1) The materials of the Earth must manifestly influence 

 its general structure, and no inquiries with this structure can 

 be usefully made if the physical properties of these materials 

 are not kept in view. If the interior of the Earth is in a fluid 

 state it is reasonable to believe that the fluid is not the ideal 

 substance called by mathematicians a perfect liquid, namely 

 a substance not only endowed with perfect mobility among 

 its particles but also absolutely incompressible. It is more 

 reasonable to believe that the fluid in question resembles the 

 liquid outpourings of volcanoes, or at least some real and 

 tangible liquid whose properties have been experimentally 

 studied. I have already shown that by overlooking this 

 simple principle certain untenable conclusions, which assert 

 the exclusively solid character of the Earth, have been deduced. 

 Here I propose to develop some additional arguments re- 

 lative to one of the properties of liquids which has an essential 

 bearing upon the internal structure of the Earth. 



(2) In a former paper, on the limits of hypotheses regard- 

 ing the properties of matter composing the Earth's interior*, 

 I find that, having referred to published statements where the 



* Philosophical Magazine for October 1878, p. 265. 

 K2 



