January, 1906.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



327 



WKslI is qlI the Centre 

 of the Eacrth? 



We happen to have receivod recently tliiee different 

 articles bearing on this subject, and, considering the 

 interest, the mystery, and the importance of the ques- 

 tion, we think it desirable to publish them in series, and 

 hope they may elicit comments and criticisms from other 

 writers. 



It seems remarkable that though we are able to 

 in\estigate the composition of stars millions of miles 

 away, and though (with the exception of a few com- 

 parativel<y small patches) we know every inch of the 

 surface of the globe, and have investigated its composi- 

 tion to the smallest particular, yet we have not he 

 slightest idea of either the chemical composition or 

 physical state of what exists but a very few miles be- 

 neath our feet. 



The diagram given below, which is taken from the 

 Science Year Book, illustrates this in a most convincing 

 manner. A glance at this is sufficient to make us doubt 

 the possibility of the theory very generally held that the 

 temperature increases so rapidly that substances of the 

 earth at so small a depth from the surface as 30 miles, or 

 only ji^ of the diameter, is at a heat sufficient to melt 

 the most refractory substances known. 



There are but few data to go upon. It is generally 

 supposed that the earth weighs about 5^ times as much 

 ;is would an equal bulk of water. The specific gravity 

 of granite, of which the greater part of the knozvn world 

 seems to consist, is only 2.6. But the pressure of the 

 superincumbent strata would be so enormous on the 

 underlying portions a few hundred miles down as to 

 greatly increase its density and raise its melting point. 

 But this pre-supposes the earth to be solid throughout. 

 If the interior be hollow, many other considerations 

 come in. We shall see, however, whether theories of 

 a hollow interior can be considered tenable. 



The Interior of the Earth. 



By BiiKEsioRD Ingram, B.A., I'.C.S. 



It is generally conceded that the present shape of the 

 globe is due to the fact that at one time the earth was a 

 .semi-tluid mass which revolved roLuid its axis with a 

 definite uniform velocity. 



Kxperiments have been conducted with spheres of 

 plastic material rotating with different velocities round 

 their axis. These have proved indubitably that, under 

 the abo\e conditions, a flatness is always produced at 

 the two extremities at which tlie axis emerges, while a 

 corresponding increase in diameter is observed mid- 

 way between these two extremes. It was also noticed 

 that the greater the velocity of rotation, the more 

 oblale the mass became. 



.\everllieless, there are, in this, man} experimental 

 considerations which would not be applicable to the 

 earth in its earliest history; so that while the plausibility 

 ol the demonstration nuist be fully admitted, it would 

 be unwise to accept the su[)position as conrhisi\elv 

 established. 



On the oilier hand, there does exist absolute proof 

 that the earth is a cooling liody. 'I he high tempera- 

 tures that have been registered at the bottom ol' mines 

 and deeper borings pro\e the point beyond all doubt 

 that, as the centre of the earth is approached, the 

 temperature becomes higher. Heat must, therefore, 

 be tra\clling from the interior to the surface, and thus 

 daily dissipated, in almost inconceivable quantities, into 

 space. 



Taking the average of all the obser\ations, it would 

 appear that the temperature rises i" I'ahr. for every 

 51 feet of the earth's crust penetrated. It follows that, 

 at a dislani-e of less than two miles, the lem]X'ratur<- is 



the same as that of boiling water; while at a distance 

 of 30 miles below the surface, the temperature is suffi- 

 cient tO' melt iron ; in fact, at a distance of 50 miles the 

 temperature is higher than any that has yet been arti- 

 ficially produced. There still remains the best part of 

 4,000 miles to travel before the earth's centre is reached. 

 It would, therefore, be meaningless to calculate the 

 temperatures that exist below fifty miles. 



The existence of enormous temperatures naturally 

 raises the question, What is the condition that prevails 

 in the interior? Is the rock in a lic|uid condition, or 

 c;ui it conceivably be in any other state? 



On this point geologists seriously disagree. 



Undoubtedly as the earth's crust is penetrated a tem- 

 perature must he reached which is sulficient to melt 

 solid rock under ordinary circiunstances, but it is 

 equally certain that at that depth an enormous pressure 

 is affecting the underlying i;iass. \ow it is well known 

 that if the pressure on a solid is increased the melting 

 point of that solid becomes higher, and, moreover, that 

 different substances are differently affected by the in- 

 crea.se in pressure. 



As it is not known what is the composition of the 

 rocks that exist at these great depths it becomes im- 

 possible to do anything but conjecture what effect the 

 pressure has on the melting point. 



When the pressure is relieved in :mv wav (as is sup- 

 posed to have taken place in volcanic outpourings of lava) it 

 would appear that the condition necessary to keep the 

 rock in a solid state is destroyed, with the observed 

 result. 



These considerations have led some scientists to con- 

 sider the earth as a solid mass with its interior at a very 

 high temperature. 



But it may happen that the increase in pressure at 

 certain depths is not sullicient to prevent the liquefac- 

 tion of the rock, in which case the earth would have a 

 solid crust with a liquid interior. 



It may In-, however, that, after a certain depth, the 



