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 obvious 

 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 from 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 equivalent to half the time taken by 

 the moon to revolve round the earth. 



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

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



