10 



THE SEA SHORE 



than that for the water on the opposite side, the earth is pulled 

 away, as it were, from the water on that side, causing it to bulge 

 out there also. Hence high tides are produced on two opposite 

 sides of the earth at the same time, while the level of the water is 

 correspondingly reduced at two other parts at right angles with 

 these sides. 



This being the case, how are we to account for the observed 

 changes in the level of the sea that occur every day on our shores ? 



Let us first see the exact nature of these changes : At a certain 

 time we find the water high on the beach ; and, soon after reach- 

 ing its highest limit, a gradual descent takes place, generally 

 extending over a period of a little more than six hours. This is 

 then followed by another rise, occupying about the same time, and 



FIG. 7. ILLUSTRATING THE TIDE-PBODUCING INFLUENCE OF THE MOON 



the oscillations are repeated indefinitely with remarkable regularity 

 as to time. 



Now, from what has been previously said with regard to the 

 tidal influence of the moon, we see that the tide must necessarily 

 be high under the moon, as well as on the side of the earth directly 

 opposite this body, and that the high tides must follow the moon 

 in its regular motion. But we must not forget that the earth itself 

 is continually turning on its axis, making a complete rotation in 

 about twenty-four hours ; while the moon, which revolves round 

 the earth in about twenty-eight days, describes only a small portion 

 of its orbit in the same time ; thus, while the tidal wave slowly fol- 

 lows the moon as it travels in its orbit, the earth slips round, as it 

 were, under the tidal wave, causing four changes of tide in approxi- 

 mately the period of one rotation. Suppose, for example, the earth 

 to be performing its daily rotation in the direction indicated by the 



