Physical geography. By Mary Somerville ...

CHAP. XVI. THE OCEAN: ITS TIDES. 201
state to a much higher latitude than if it had been fresh, while it
is better suited for navigation by its greater buoyancy. The healthfulness of the sea is ascribed to the mixing of the water by tides
and currents which prevents the accumulation of putrescent matter.
Besides its saline ingredients, the sea contains bromine and iodine
in very minute quantities, and, no doubt, portions of other substances too small to be detected by chemical analysis,' since it has
constantly received the debris of the land and all its organised
matter.
Raised by the moon and modified by the sun, the area of the ocean
is elevated into great tidal waves which keep time with the attractions of these luminaries at each return to the upper and lower meridian. The water under the moon is drawn from the earth by her
attraction, at the same time that she draws the earth from the water
diametrically opposite to her, in both cases producing a tide of nearly
equal height. The height to which the tides rise depends upon the
relative positions of the sun and moon, upon their declination and
distance from the earth, but much more upon local circumstances.
The spring tides happen at new and full moon, consequently, twice
in each lunar month, because in both cases the sun and moon are in
the same meridian; for when the moon is new they are in conjunction, and when she is full they are in opposition, and in each of
these positions their attraction is combined to raise the water to its
greatest height; while, on the contrary, the neap or lowest tides
happen when the moon is in quadrature, or 90~ distant from the
sun, for then they counteract each other's attraction to a certain
degree.
The tides ordinarily happen twice in 24 hours, because the rotation of the globe brings the same point of the ocean twice under the
meridian of the moon; but peculiar local circumstances sometimes
affect the tides, so as to produce only one tide in 24 hours, while on
the other hand there have been known three and even four tides in
the same space of time.
As the earth revolves, a succession of tides follow one another,
and are diffused over the Pacific, Indian, and Atlantic Oceans, giving birth to the tides which wash the shores of the vast continents
and islands which rise above their surfaces; but in what manner
these marginal tides branch off from the parent wave, science has
not yet determined: we know only their course along each shore,
but are unable to connect these curves with the great ridge of the
tidal wave.
In the Atlantic the marginal wave travels towards the north, and
impinges upon the coasts of North America and of Europe. In
1 t has been recently stated by a very learned chemist, M. Malaguti,
that sea-water contained silver in very minute portions. -'Comptes
Rendus,' 1849-50.