Foundations of the Universe x'!>l 



of mound. The whole earth is pulled by the moon, but the water 

 is more free to obey this pull than is the solid earth, although 

 small tides are also caused in the earth's solid crust. It can he- 

 shown also that a corresponding hump would tend to be produced 

 on the other side of the earth, owing, in this case, to the tendency 

 of the water, being more loosely connected, to lag behind the solid 

 earth. If the earth's surface were entirely fluid the rotation of 

 the earth would give the impression that these two humps were 

 continually travelling round the world, once every day. At any 

 given part of the earth's surface, therefore, there would be two 

 humps daily, i.e. two periods of high water. Such is the simplest 

 possible outline of the gravitational theory of the tides. 



The actually observed phenomena are vastly more compli- 

 cated, and the complete theory bears very little resemblance to the 

 simple form we have just outlined. Everyone who lives in the 

 neighbourhood of a port knows, for instance, that high water 

 seldom coincides with the time when the moon crosses the 

 meridian. It may be several hours early or late. High water 

 at London Bridge, for instance, occurs about one and a half 

 hours after the moon has passed the meridian, while at Dublin 

 high water occurs about one and a half hours before the moon 

 crosses the meridian. The actually observed phenomena, then, 

 are far from simple ; they have, nevertheless, been very completely 

 worked out, and the times of high water for every port in the 

 world can now be prophesied for a considerable time ahead. 



The Action of Sun and Moon 



It would be beyond our scope to attempt to explain the com- 

 plete theory, but we may mention one obvious factor which must 

 be taken into account. Since the moon, by its gravitational attrac- 

 tion, produces tides, we should expect that the sun, whose gravi- 

 tational attraction is so much stronger, should also produce tides 

 and, we would suppose at first sight, more powerful tides than 

 the moon. But while it is true that the sun produces tides, it is 



