458 



SCIENCE. 



ON THE INTERIOR CONDITION OF THE TER- 

 RESTRIAL GLOBE. 

 By M. E. Roche. 



Generally we admit that the earth is entirely fluid in 

 its interior, with the exception of a thin crust, and most 

 of the mathematical studies on the figure and the consti- 

 tution of the earth assume this fluidity. Thus by attrib- 

 uting to this fluid a certain law of compression, Laplace 

 has deduced a corresponding law of densities, which 

 Legendre had already examined before him, and which 

 permits the calculation of the flattening of the different 

 surfaces de tiiveau of the terrestrial mass. I have my- 

 self proposed another law of compressibility, which con- 

 ducts to a very simple formula for the increment of the 

 density. The conditions which every hypothesis must 

 satisfy, on the distribution of the mass in the interior of 

 the earth, are that it must accord with the value of the 

 superficial flattening, and also with a certain constant 

 depending on the phenomenon of precession. These 

 conditions are very approximately satisfied in the hypo- 

 thesis of fluidity, if we admit that the terrestrial flattening 

 is nearly ^in; but if this flattening is greater than as 

 it seems to result from the most recent determinations, 

 the agreement no longer exists. 



There is need, therefore, of a new examination of these 

 researches under a different hypothesis, for example, in 

 considering the globe as formed of a nucleus or solid 

 mass very nearly homogeneous, covered with a lighter 

 shell whose density, from geological considerations, can 

 be estimated as 3 with respect to water. This constitu- 

 tion of the globe being supposed, I find that it is possible 

 to conciliate the actual values of the precession and of 

 the flattening, if we take account of this, that the inter- 

 ior nucleus of the globe solidified and has taken its defin- 

 itive form under the influence of a rotation less rapid 

 than that with which the earth is now animated. 



In every case the contraction due to this cooling of 

 the globe must lead to a progressive acceleration of 

 its angular velocity. But if this globe is fluid the figure 

 of the different strata adapts itself continually to the ro- 

 tation which has place at each instant, in such a way 

 that finally there remains no trace of the successive vari- 

 ations which their flattening have undergone since the 

 origin. If, on the contrary, at a certain epoch of the 

 cooling the interior strata have passed to a solid state, 

 these strata have taken and preserved a flattening very 

 different from that which would be attributed to them 

 by the general equation of hydrostatics applied to a mass 

 entirely fluid and possessing a rotation common to all its 

 parts. The formulae calculated in the hypothesis of a 

 solid nucleus contains at the same time the constant q, 

 the actual ratio of the centrifugal force to the equa- 

 torial gravity, and value q,„ of the same ratio at the epoch 

 of the solidification of the central mass. This last ele- 

 ment, not being determined, we can give to it a value 

 such that the superficial flattening accords with the co- 

 efficient of precession. It is necessary for this to suppose 

 q u less than q, whence it results that the terrestrial ro- 

 tation has undergone an acceleration since the consolida- 

 tion of the interior nucleus. 



The physical and astronomical conditions of the prob- 

 lem permit also the determination, with some precision, 

 of the dimensions and specific weight of this central mass. 

 If we leave out of consideration the crust purely super- 

 ficial, as also a slight condensation towards the centre 

 where the heavier materials would be collected, the con- 

 stitution of the globe will be as follows : a nucleus, of 

 which the density is nearly 7, covered with a shell of 

 density 3, whose thickness does not attain one-sixth of 

 the entire radius. 



The central terrestrial mass is therefore in specific 

 weight analogous to meteoiic iron, while the stratum, 

 that envelops it is comparable to aerolites of a stony 

 nature, where iron enters only in a small proportion. 



BOOKS RECEIVED. 



The Microscope and its relation to Medicine 

 and Pharmacy. Edited by Chas. H. Stowell, 

 M. D., and Louisa Reed Stowell. Published 

 monthly. Ann Arbor, Michigan, 1881. 



There are already two journals published monthly 

 which are devoted to microscopy, it is therefore with 

 some surprise that we find a third journal of the same 

 description appealing to the patronage of Microscopists. 



"The Microscope " claims to supply a want in offering 

 physicians a journal which treats exclusively of Medical 

 and Pharmaceutical Microscopy, thus differing from the 

 two former microscopical journals which cover the whole 

 field of Microscopy. 



We believe that the success of " The Microscope" 

 will depend upon that journal being conducted strictly 

 within the limits of its own programme. Undoubtedly 

 the majority of American microscopists are members of 

 the medicil profession and, therefore, "The Microscope" 

 may look for a numerous constituency. 



" The Microscope" has been produced in an excellent 

 form, is well printed, and illustrated with good illustra- 

 tions, and if the editors will confine the columns to Mi- 

 croscopy, to the exclusion of facetious " items " clipped 

 from their exchanges, they may hope to place their 

 journal on a firm basis. 



CORRESPONDENCE. 



[The Editor does not hold himself responsible /or opinions expressed 

 by his correspondents. No notice is taken of anonymous communi- 

 cations.'] 



To the Editor of " Science." 



Permit me to suggest a few questions that should be 

 answered by the author of " The Great Primordial 

 Force." (See " Science," p. 405.) 



Let me introduce my questions by announcing my 

 belief in the conservation of energy and in the unity of 

 force and by expressing a hope that the time will come 

 when the phenomena peculiar to the different manifesta- 

 tions of force may be comprehended and their identity 

 demonstrated. 



The following questions are respectfully suggested : 



1. When, and by whom, has it been demonstrated that 

 gravitation is resolvable into light or heat? 



2. If all force " is substance," or matter, (see "Science," 

 p. 405, last paragraph), then, at least some matter is 

 force. What then is the distinction between matter and 

 force ? 



3. On p. 406, it is asserted that the two elements, 

 " Motion and Magnetism," "develop all known forces of 

 the universe." How can motion exist without a prior 

 force, and what is magnetism ? 



4. What produces the revolution of the revolving celes- 

 tial armatures ? 



The foregoing questions may be sufficient for the 

 present, but it is due to science that statements claiming 

 to be scientific should be truths. Let us examine some 

 of the author's statements. 



It is asserted that " if gravity acts inversely as the 

 square of the distance, then the earth at aphelion could 

 not, without the aid of some other fotce, return to peri- 

 helion." It is to be feared that the author of the above 

 question is not aware that the inertia of matter is an ex- 

 perimental fact, and is entirely sufficient to bring back 

 the planet from aphelion to perihelion. 



As to the law of inertia, all matter, if not acted upon 

 by some external force, will continue in its present state, 

 whether of motion or rest, and because while a planet is 

 passing from perihelion to aphelion the tangent to its 

 orbit makes an obtuse angle with its radius vector ; 

 therefore its inertia counteracts, and to a certain extent, 



