OUR PRESENT KNOWLEDGE OF THE EARTH." 



By E. WlECHERT,^ 



Professor of Geophysics at the University of Gottingen. 



The exiDlorer of nature is naturally inclined to direct his search to 

 the earth itself, on whose surface we live. He asks what secrets may- 

 lie hidden in the depths beneath our feet. My purpose on the pres- 

 ent occasion is to set forth some of the answers which science is now 

 able to give to that question. 



The simplest method, of course, would be for the explorer himself 

 to penetrate into the earth by the way pointed out by the miner. But 

 this hope quickly vanishes as we survey the means at our disposal 

 and the results thus far achieved. Mining operations extend to a 

 depth of about 1 kilometer (3,280 feet) ; the deepest shaft ever bored 

 reached a depth of about 2 kilometers (6,560 feet), and the center 

 of the earth is 6,370 kilometers (about 4,000 miles) beneath us. 



Wliat are 2 kilometers compared to that? Imagine the earth rep- 

 resented by a ball 13 meters (42 feet) in diameter; then a shaft 2 kilo- 

 meters (6,560 feet) deep would be represented by a needle prick 

 2 millimeters (about one-twelfth inch) deep! And yet the sinking of 

 such a shaft is a work of exceeding difficulty; with every meter the 

 difficulties increase at an accelerated rate, soon outstripping all human 

 power. 



Thus in our search into the interior of the earth, apart from a very 

 thin superficial layer, we must depend entirely on the resources of 

 science. 



Immediately beneath us we find masses of rock ; that we know for 

 certain. But what is there farther down? Does the rock continue 

 throughout all the depths, or shall we come to metals ? Rocks are com- 

 paratively light, the metals that might be expected are comparatively 

 heavy. Hence if the density of the earth's material is known, we 



'^Translated by perruissiou from Deutsche Rundschau, Band CXXXII (Sep- 

 tember, 1907), Berlin, pp. 376-394. 



^ Address delivered for the benefit of the Gottingen Ladies' Union, February 

 14, 1907. 



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