12 



TEE SEA SHOEE 



not frequently visible. Careful observations of the motions of tho 

 water for some days after will show that this great difference 

 between the levels of high and low- water gradually decreases until, 

 about a week later, it is considerably reduced, the high tide not 

 flowing so far inland and the low-water mark not extending so far 

 seaward. Then, from this time, the difference increases again, till, 

 after about two weeks from the commencement of our observations, 

 we find it at the maximum again. 



FIG. 9. SPUING TIDES AT FULL MOON 



Here again we find that the changes exactly coincide with 

 changes' in the position of the moon with regard to the sun and 

 the earth. Thus, the spring tides those which rise very high 

 and fall very low always occur when the moon is full or new ; 

 while the less vigorous neap tides occur when the moon is in her 

 quarters and presents only one-half of her illuminated disc to the - 



FIG. 10. SPRING TIDES AT NEW Moon 



earth. And, as the moon passes through a complete cycle of 

 changes from new to first-quarter, full, last- quarter, and then to 

 new again in about twenty-nine days, so the tides run through four 

 changes from spring to neap, spring, neap, and then to spring again 

 in the same period. 



The reason for this is not far to seek, for we have already seen 

 that both sun and moon exert a tide-producing influence on the 



