PEESENT KNOWLEDGE OF THE EARTH WIECHEET. 445 



render to the geophysicist somewhat the same service that the Ront- 

 gen rays render to the physician who tries to examine the human body. 



If we consider the feebleness of human means, natural science may 

 well be proud of its achievements. But we should not forget also 

 that the retrospect affords us no less motive for humility ; for, as I 

 tried to point out, success was only achieved by an immense expendi- 

 ture of labor and pains. And yet I have reported to you only the 

 advances, and have said nothing of the failures and mistakes, nothing 

 of the labor that was spent in vain. 



In fact, in this respect geophysics does not differ from the other 

 branches of natural science — every forward step which man is able to 

 make in the path of knowledge is dearly purchased. But this need 

 not discourage us. So long as the jo}^ of life fills the breast of man 

 conquerors will be found ready, not as dreamers but with the will to 

 do, placing their strength resolutely and joyously at the service of 

 the investigation of nature. 



I will add by way of supplement a few remarks additional to those 

 made in the address. 



(a) The moon a drop of the earth'' s crust. — To man, standing on 

 the surface of the earth, the tide waves seem to run around the earth 

 with the moon. But if we imagine an observer stationed in space he 

 will have the impression that the earth revolves beneath the tide 

 waves created and held in position by the attraction of the moon and 

 sun. As the tidal currents involve friction of various kinds, it is 

 evident that the earth must be checked in its revolution. The re- 

 tardation is but slight ; calculating from the present factors, we infer 

 that it would merely add one second to the length of the day in about 

 five hundred thousand years. However, looking back into the past, we 

 find that many millions of years ago the earth revolved far more 

 rapidly than it does now. The tidal friction has another consequence 

 of importance for our inquiry: It causes the moon to move farther 

 and farther awaj^^ from the earth. To understand how this comes 

 about we must note that the rotating earth drives the tide waves 

 somewhat forward, in advance of the moon. As a consequence, the 

 tide waves, by reciprocal gravitation, impart to the moon a constant 

 forward impulse, and this acceleration, which means an increased 

 force of revolution, drives the moon away from the earth. 



If, with our minds fiJled with the ideas thus gained, we go back to 

 remote ages, we arrive, by the aid of calculation on the basis of actual 

 conditions, at a time when the earth performed one revolution in a 

 few hours, while the moon revolved around it in immediate proximity, 

 in the same space of time. This evidently leads us back to the time 

 when the moon itself originated by detachment from the earth. The 

 process must have been somewhat as follows: In the early ages the 

 earth in its outer parts was liquid, or even gaseous. Contraction, due 



