^067'^/ Life in the Salt-JJ'ater World 19 



its axis, a complete rotation occurring every twenty- 

 four hours; and as the tides necessarily follow the 

 moon which in itself revolves around the earth approxi- 

 mately every twenty-eight days the changes in the tides 

 occupy on the average six hours and a quarter each; 

 thus making high and low tides occur about an hour 

 later every day. 



Now in all strictness the influence of the sun is not so 

 negligible a factor as it may appear from the fore- 

 going. Although its attraction is considerably less than 

 that of the moon, it is still an attraction. This we 

 find is evident in the fortnightly fluctuations of the 

 tides. As was pointed out, extreme high and low tides 

 are at the time of the new and the full moon, whereas 

 the least variation is during the quarter phases. The 

 former are known as "spring tides" ; the latter as 

 "neap tides." As we follow the moon in its various 

 phases, we find in starting with the new moon that it 

 is nearly on a straight line — sometimes exactly so — 

 between the earth and the sun; therefore, the attrac- 

 tion of the sun added to that of the moon, assists the 

 latter, as it were, in its pull, thus causing a spring tide. 

 About a week later when the moon is in its first quarter 

 it is in a position at right angles from the sun as viewed 

 from the earth. The tides are now more modified, for 

 the moon no longer has the help of the sun. The truth 

 is, each of these bodies is trying to produce a tide all 

 iis own, the moon on one meridian and the sun on 

 another, but a quarter of the way around the earth; 

 the consequence is that though the moon's pull is 



stronger its efforts are minimized by the sun's power 



in holding the water in check. A neap tide is the resuffi^((J^|^ 



[!±f (librar\ 



