PRESENT KNOWLEDGE OF THE EARTH WIECHERT. 437 



much as two flood tides are moving around the earth, the tide 

 phenomena recur at intervals of about twelve and one-half hours. 



I have tried to set forth to you the fundamental facts of the tide 

 phenomenon, but I must add that in reality the process is exceedingly 

 complicated. On the one hand, astronomic factors enter into the 

 problem, for example, the fact that both the sun and the moon are 

 now farther north and again farther south; on the other hand, the 

 irregidarities in the distribution of water and land on the earth lead 

 to extensive modifications in the course of the tides which are difficult 

 to estimate. How strong these influences are you may gather from 

 the fact that the tides on the shores of the Xorth Sea come from the 

 Indian Ocean. South of America, Australia, and Africa there is a 

 broad belt of open water; there the tide waves develop freely. Start- 

 ing from the Pacific they run through the Indian Ocean, and as they 

 enter thence into the Atlantic Ocean south of Africa, a part of each 

 tide wave is deflected northward into the Atlantic Ocean. In the 

 course of a little more than twelve hours these partial waves reach 

 England and then pass around it on the south and north into the 

 North Sea. The velocity becomes slower and slower with decreasing 

 depth. After issuing from the Indian Ocean it takes the tide about 

 two days to reach our German coasts. 



Now, what does the tide teach us regarding the condition of the 

 globe? If the earth were entirely plastic — that is to say, if the inte- 

 rior were in the main in the liquid or even the gaseous condition, as 

 has been assumed from time to time by the imagination of some sci- 

 entists — there could be no ebb or flood. The earth's body itself would 

 in that case be deformed under the varj'ing attraction of sun and 

 moon, and the result would be that a relative movement of the sea 

 such as is indicated by the rise and fall of the tide could not take 

 place. Hence, the existence of tides proves that the earth acts in the 

 main as a solid hodj. 



Thus there can be no doubt that the earth possesses a certain power 

 of resistance to changes of form. But what is the extent of this 

 power? Combining observation and calculation, we can draw an 

 inference on this point also. The ordinary half-daily tide, indeed, 

 can not be used for that purpose, because, in regard to this, it would 

 be too difficult to form a correct estimate of the influence of the irregu- 

 larities in the distribution of land and water. But the half-monthly 

 tide connected with the movement of the moon from north to south 

 and back, in its revolution around the earth, may be utilized, for in 

 this case, owing to the slowness of the process, the sea has time to 

 follow the varying forces of attraction without disturbing current 

 phenomena. This half-monthly tide, indeed, is very small, but in 

 view of the extreme precision with which, in the interest of navi- 

 gation, the changes of sea level are observed and used for calculation, 



