GENERAL CHARACTERISTICS 9 



nical theories of the tides, for which the reader should refer to 

 authoritative works on the subject, but merely endeavour to briefly 

 explain the observed oscillations of the sea and the general laws 

 which govern them. 



The most casual observer must have noticed the close connec- 

 tion between the movements of the ocean and the position of the 

 moon, while those who have given closer attention to the subject 

 will have seen that the relative heights of the tides vary regularly 

 with the relative positions of the sun, moon, and earth. 



In the first place, then, we notice that the time of high tide in 

 any given place is always the same at the same period of the cycle 

 of the moon ; that is, it is always the same at the time of new 

 moon, full moon, &c. Hence it becomes evident that the moon is 

 the prime mover in the formation of tides. Now, it is a fact that 

 the sun, though about ninety-three millions of miles from the 

 earth, has a much greater attractive influence on the earth and its 

 oceans than the moon has, although the distance of the latter is 

 only about a quarter of a million miles : but this is due to the 

 vastly superior mass of the sun, which is about twenty-six million 

 times the mass of the moon. How is it, then, that we find the 

 tides apparently regulated by the moon rather than by the sun ? 



The reason is that the tide-producing influence is due not to 

 the actual attractive force exerted on the earth as a whole, but to 

 the difference between the attraction for one side of the globe and 

 that for the opposite side. Now, it will be seen that the diameter 

 of the earth about eight thousand miles is an appreciable frac- 

 tion of the moon's distance, and thus the attractive influence of the 

 moon for the side of the earth nearest to it will be appreciably 

 greater than that for the opposite side ; while in the case of the 

 sun, the earth's diameter is such a small fraction of the distance 

 from the sun that the difference in the attractive force for the two 

 opposite sides of the earth is comparatively small. 



Omitting, then, for the present the minor tide-producing influ- 

 ence of the sun, let us see how the incessant rising and falling of 

 the water of the ocean are brought about ; and, to simplify our 

 explanation, we will imagine the earth to be a globe entirely 

 covered with water of uniform depth. 



The moon attracts the water on the side nearest to it with a 

 greater force than that exerted on the earth itself ; hence the water 

 is caused to bulge out slightly on that side. Again, since the 

 attractive force of the moon for the earth as a whole is greater 



