436 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1908. 



were merely an effect of pressure and not of difference in material. 

 It is exceedingly fortunate, therefore, that our conclusions receive new 

 and very strong support from another and entirely different direction, 

 that of earthquake investigation. But before I proceed to explain 

 this, I must refer to two other phenomena of nature, whose data may 

 be utilized in drawing conclusions concerning the condition of the 

 interior of the earth. 



I refer to the question of the plasticity of the earth under the in- 

 fluence of deforming forces. Aside from the phenomenon of earth- 

 quakes, there are two natural processes that bear on the question — • 

 (1) tides, (2) polar oscillations. 



The tide, that " breathing of the sea," is to j^ou a familiar phe- 

 nomenon in its main features. Many of you have doubtless been 

 eye witnesses of it on the shores of the North Sea. The causes of 

 the tides are easily understood ; they are to be sought in the attraction 

 of sun and moon. Each of these heavenly bodies attracts the water 

 of the sea more strongly on the side of the earth nearer to it than it 

 does the earth's body as a whole, while on the opposite side of the 

 earth it attracts the water of the sea more feebly than it does the body 

 of the earth, which in this case is nearer to the attracting body. The 

 stronger attraction on the near side, as may readily be seen, produces 

 an upheaving of the water — that is to say, a flood tide; but there is 

 also a flood tide on the off side of the earth, because, since the water 

 there is more feebly attracted than the earth's body as a whole, it 

 assumes, in the course of the earth's movement in space, a position 

 relatively more distant from the attracting body than the earth, 

 which means nothing else than that it rises in the form of a flood 

 tide, relatively to the earth. Thus both the sun and the moon are 

 each accompanied by two flood tides, one on the near side, the other 

 on the off side of the earth; and in view of the revolution of the 

 earth and the relative movements of the heavenly bodies, the result 

 is that to the observer on the earth's surface sun and moon arc each 

 followed in their course around the earth by two flood tides. The 

 moon is so much closer to the earth than the sun that despite its 

 smaller mass the tides caused by it are more than twice as large as 

 those caused by the sun. As a consequence, the sun tides do not pre- 

 sent themselves as separate from the moon tides, but merely as modi- 

 fications of them. At new moon and full moon the sun's tide reen- 

 forces the moon's tide, and we have the so-called " spring tides." At 

 half mopn the sun's tide is. opposed to the moon tide, and then we 

 have the " neap tides." At any rate, to the observer the tides seem 

 always to follow the moon in its course through the heavens. Be- 

 cause of the daily revolutions of the earth around its axis and the 

 moon's own motion about twenty-five hours pass before it has accom- 

 plished apparently one revolution around the earth, and thus, inas^ 



