GENERAL CHARACTERISTICS 17 



place is always approximately the same during the same relative 

 positions of the sun, earth, and moon ; that is, for the same ' age ' 

 of the moon ; so that it is possible to determine the time of high 

 water at any port from the moon's age. 



The time of high tide is generally given for the current year in 

 the local calendars of our principal seaports, and many guide-books 

 supply a table from which the time may be calculated from the age 

 of the moon. 



At every port the observed high water follows the meridional 

 passage of the moon by a fixed interval of time, which, as we have 

 seen, varies considerably in places within a small area of the globe. 

 This interval is known as the establishment of the port, and pro- 

 vides a means by which the time of high water may be calculated. 



Before closing this short chapter on the general characteristics 

 of the sea shore we ought to make a few observations on the 

 nature of the water of the sea. Almost everyone is acquainted 

 with the saltness of this water as revealed by its decided taste, 

 while many bathers have noticed the superior buoyancy of salt 

 water as compared with the fresh water of our rivers and lakes. The 

 dissolved salts contained in sea water give it a greater density than 

 that of pure water ; and, since all floating bodies displace their own 

 weight of the liquid in which they float, it is clear that they will 

 not sink so far in the denser water of the sea as they would in 

 fresh water. 



If we evaporate a known weight of sea water to dryness and 

 weigh the solid residue of sea salt that remains, we find that this 

 residue forms about three and a half per cent, of the original 

 weight. Then, supposing that the evaporation has been conducted 

 very slowly, the residue is crystalline in structure, and a careful 

 examination with the aid of a lens will reveal crystals of various 

 shapes, but by far the larger number of them cubical in form. 

 These cubical crystals consist of common salt (sodium chloride), 

 which constitutes about three -fourths of the entire residue, while 

 the remainder of the three and a half per cent, consists principally 

 of various salts of magnesium, calcium, and sodium. 



Sea salt may be obtained ready prepared in any quantity, as it 

 is manufactured for the convenience of those who desire a sea bath 

 at home ; and it will be seen from what has been said that the 

 artificial seawater may be prepared, to correspond almost exactly 

 with that of the sea, by the addition of three and a half pounds of 

 sea salt to about ninety-six and a half pounds of water. 



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