PRESENT KNOWLEDGE OF THE EARTH WIECHERT. 433 



its surface, we also Ivnow the size of the earth and therefore the dis- 

 tance between its various parts and the bodies on its surface ; and thus 

 the laboratory observations on the mutual attraction of bodies enable 

 us to calculate the total mass of the earth. We find that the total mass 

 is five and a half times greater than it would be if the entire space 

 occupied by the earth were filled with water; in other words: The 

 various kinds of substances contained in the earth are on an average 

 five and a half times denser than water. This figure, 5|, is the cul- 

 mination of all the observations on gravitation of which I speak. 

 But in this number, you will understand, the physicist also perceives 

 the outcome of a sum total of human labor, by which one of the forces 

 of nature dominating the world has been brought nearer to human 

 i:)erception. 



The rocks on the earth's surface are only 2^ to 3^ times heavier 

 than water ; hence, since the earth is on an average 5^ times heavier 

 than water, it follows that at a certain depth the density must be 

 greater than 5|. It has been suggested that this greater density may 

 simply be an effect of the pr&ssure exerted by the overlying layers 

 of the earth on the underlying. I have never been able to subscribe 

 to that view, since everything that we know concerning the constitu- 

 tion of matter, and its construction out of the highly resistant atoms, 

 indicates, it seems to me, that a notable compression by the pressure 

 of the earth is not to be expected. Thus it seems to me that the 

 greater density in the deeper depths of the earth's interior can only 

 be explained by assuming that lieavier substances, especially metals, 

 predominate there. At any rate, you see that we are here in a state 

 of uncertainty. AVe are compelled to ask. Are there not other means 

 by which the distribution of mass in the body of the globe may be 

 inferred? In point of fact, they may be found. First of all, there 

 are the observations on the shape of the earth. 



You know that the earth — assuming its surface to be represented 

 by the surface of the sea, which we conceive as extending also through 

 the continents — is not an exact sphere, but has been flattened at the 

 poles, owing to the centrifugal force of its rotation. This flattening 

 is of the greatest importance for general geophysics and for all esti- 

 mates concerning the mutual position of points on the earth's surface. 

 It must be taken into account in all land measurements, in all surveys 

 intended to ascertain the geographic distribution of lands. Such 

 geodetic observations form also the basis of the calculations by which 

 we ascertain the size of the earth. The magnitude of the flattening 

 also manifests itself in the distribution of gravitation on the earth's 

 surface. That this ma}'- be the case you will readily understand, if 

 you consider that it is gravitation that determines the shape of the 

 surface of the sea. It has been found that gravitation becomes 

 greater the farther we remove from the equator and approach the 



