4 SCIENCE PROGRESS 



forces set up in it by passing near to another star. The 

 assumed partial disintegration thus effected in the parent star 

 and the mode in which subsequently aggregation of the ejected 

 matter into planets and satellites might have taken place offer 

 problems so intricate as to defy calculation ; the indications that 

 can be obtained certainly seem to suggest that we have in this 

 theory a modification of Laplace's free from many of the difficulties 

 inherent in his original form. 



Earth Tides 



The body of the Earth, on which the oceans rest, cannot 

 be absolutely rigid. No body is. It must be deformed more 

 or less by the attractions of the Sun and Moon. If we can find 

 out in what manner it is deformed and how much, we can draw 

 inferences in regard to its internal constitution. Thus there 

 arises the problem of earth-tides : How can such tides be ob- 

 served ? What conclusions in regard to the state of the matter 

 within the Earth can be drawn from the observations ? The 

 movement eludes direct observation. A tide-gauge can record 

 the rise of water above a marked level near a coast and other 

 instruments can do the same thing for the rise above a level 

 measured from the sea-bottom out at sea but the would-be 

 observer of earth-tides has no mark from which to measure. 

 His methods of observation must perforce be indirect. The 

 first attempts were directed to finding the actual height of the 

 so-called fortnightly tide. By the fortnightly tide is meant 

 a minute inequality in the tide-height, having a period of about 

 a fortnight, depending upon the inclination of the Moon's orbit 

 to the plane of the equator. The point at which the Moon 

 is overhead is not always or generally a point on the equator 

 but travels round and round the Earth in a sort of spiral path. 

 The whole spiral lies between two extreme turns, one the most 

 northerly, the other the most southerly, which, however, are 

 not fixed but vary in position from time to time. If we follow 

 the movement of the point, beginning at an instant when it 

 has an extreme northerly position, we find each successive turn of 

 the spiral lying to the south of the preceding turn, until at the 

 end of a fortnight an extreme southerly position is reached. After 

 this the path turns to the north and during the next fortnight 

 each successive turn of the spiral lies to the north of the pre- 



