SEA WATER 237 



interval between one high tide and the next is not exactly 

 twelve hours, but on the average twelve hours and twenty 

 minutes. Thus the time of high water is not the same 

 every day, but there is a progressive change, the high tide 

 of to-morrow afternoon being about forty minutes later 

 than that of this afternoon. So throughout the years 

 there is practically no time of day at which high or low 

 water may not occur at any place. 



Together with this change in the time of the tides there 

 is also to be noticed a change in the actual range of the 

 tides, the distance between high and low-water marks 

 becoming progressively greater throughout a certain period 

 and then correspondingly smaller. These periods are 

 known as the periods of " spring " and of " neap " tides. 



During the " spring " tides (Saxon : sprungen, " to 

 bulge ") the low- water mark is lowest and the high- water 

 mark highest, that is, the range between the tide marks 

 is greatest ; during the neaps the reverse is the case, the 

 tide only coming in and going out a short distance. Besides 

 the attraction of the moon there is also a smaller attraction 

 exerted by the sun. It is because of the sun's attraction 

 that the spring and neap tides are caused. When the sun 

 and moon are both exerting their pulls in the same direction 

 the force will be at its greatest and the big tides or springs 

 will occur. This happens shortly after both new and full 

 moon (Fig. 50). But the range of the tides will be greatest 

 at new moon, because then the sun and moon are both 

 on the same side of the earth ; and the spring tides are less 

 at full moon, the sun and moon being then opposite one 

 another. When the forces of the sun and moon are at 

 right angles to one another, i.e. at the periods of half 

 moon, the range of the tides will be least and neap tides 

 will result. 



The tides must not be confused with " tidal streams " 



