THE TIDES 109 



length, whereas the originally vertical columns become elon- 

 gated in the neighbourhood of the wave crests, and shortened 

 near the wave hollows " (White). 



The Tides. 



The term " tide " is given to the periodic rise and fall of 

 the sea-surface. Naturally this rise and fall is more obvions 

 on the coast-line, and is of such a nature that the period between 

 the heights of two successive rises or falls is approximately 

 half a day. 



The highest water-level attained by a given tide is called 

 high-water, the lowest low-water ; the rise of the tide f rom the 

 lowest to the highest position is called the flood and the fall 

 from the highest to the lowest position the ebb. The perpen- 

 dicular distance between the water-levels at high and low 

 water is the range of the tide. 



If observations on the rise and fall of the tide be made for 

 a period of not less than fourteen days it will be seen that there 

 are variations both in the time at which high-water is attained 

 and the height attained by the tide at its maximum. Every 

 fourteen days the tide at high-water attains a maximum and 

 minimum height. The former is a spring-tide, the latter a 

 neap-tide. 



The duration of a " flood " — that is, the time between the 

 lowest water of a tide and the highest water of the succeeding 

 tide — is not exactly six hours, so that the interval between 

 corresponding tides in successive days is on the average 24 

 hours 40 minutes ; and this circumstance, as well as the fort- 

 nightly interval between successive spring-tides, is associated 

 with the movement of the moon. The duration of a tide is 

 almost exactly half a lunar day, while the period between 

 successive spring-tides is équivalent to half the time taken by 

 the moon to revolve round the earth. 



As a conséquence at every place or port it is noticed that 

 the time of high-water seems to follow the moon 's méridional 



