PRESENT KNOWLEDGE OF THE EARTH WIECHERT. 439 



surprising, for we do not know the early history of the earth which 

 has brought it to its present condition. But the moment we examine 

 the pole oscillations on the basis of existing observations more accu- 

 rately, we find several remarkable facts. Theorj' teaches the following : 

 If the earth were perfectly rigid, that is to say, if it were not yield- 

 ing in the slightest degree, and if no disturbances intervened, the 

 axis of revolution would have to move incessantly and with the same 

 velocity around the axis of figure, so that the astronomical poles of 

 the earth would constantly describe the same circles around the mean 

 position. Astronomy is able to indicate in what time a revolution 

 would be completed, namely, in three hundred and five days. But 

 when we consult the observations we find a totally different course 

 of the pole oscillations. There is indeed a circular movement of the 

 poles in the true sense, but it takes place not in three hundred and 

 five days, but in four hundred and twenty-five days. Moreover, the 

 curves described change their width in an apparently irregular man- 

 ner from revolution to revolution. These latter irregularities indi- 

 cate that disturbing causes are constantly at work which continually 

 displace the axis of revolution. It has been found that even the 

 meteorologic processes in the atmosphere suffice to explain these dis- 

 placements. Some geophysicists suggest that earthquakes may coop- 

 erate. Be this as it may, it is evident that the irregularities present 

 no difficulty to the explanation, and thus we need not trouble our- 

 selves about them. But what shall we say to the fact that the 

 circular path of the poles is traversed not in three hundred and five 

 but in four hundred and twenty -five days? Here we have arrived 

 at the point which invests the phenomenon of pole oscillations with 

 great significance for the question of the condition of the globe. In 

 fact, from the difference between the observed time of revolution and 

 the time ascertained by calculation under the assumption of a per- 

 fectly rigid globe, it may be inferred that the earth is not absolutely 

 rigid but plastic. Part of this plasticity is to be accounted for by 

 the movable, liquid body of the sea ; but it can be shown that this 

 accounts for only about one-fourth of the difference between three 

 hundred and five and four hundred and twenty-five days. Hence 

 the earth beneath our feet, which to our senses seems absolutely rigid, 

 must to a certain degree be yielding. It is possible to calculate this 

 degree of yielding, and we find that it is about half as great as if 

 the globe possessed the rigidity of steel. In other words, the earth 

 opposes to the deforming forces about twice the resistance that steel 

 manifests under the conditions under which we observe it in our daily 

 life. You see that the phenomenon of the pole oscillations carries 

 us much further in our inquiry into the condition of the globe than 

 does the phenomenon of the tides. From the observation of the tides 

 we were merely able to infer that the earth is at least as rigid as 

 88292— SM 1908 29 



