Although the time of high water varies so greatly on the same day over such a small area of country, yet that time for any one 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 provides 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 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 sea-water 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. This is often a matter of no little importance to the sea-side naturalist, who may require to keep marine animals alive for some time at considerable distance from the sea shore, while their growth and habits are observed. Hence we shall refer to this subject again when dealing with the management of the salt-water aquarium.