Seawater covers about 70 percent of the earth’s surface. According to where it is, it contains variable amounts of salt. The Red Sea and the Mediterranean Sea are considerably more saline than the cold waters that circulate around Antarctica. The greater part of seawater is Oxygen - almost 80 percent of it, and Hydrogen amounts to about 11 percent. Of the remaining elements, Chloride and Sodium account for over 3 percent.
The density of seawater varies according to its salinity and temperature. Hence a ship has a series of lines painted on her hull in the region of the waterline known as the Plimsoll Line or the Plimsoll Mark. These markings indicate the depth to which she should be loaded. A ship is loaded to be safe so that there is no danger of her sinking when she passes through the oceans and waterways of the world. For example, warm saltwater is less dense than cold saltwater and tropical fresh water is the least dense of all. The denser the water is, the more buoyant it is. Therefore a vessel leaving Iceland in winter, bound for the River Niger in Nigeria, in central western Africa, at most, she will be loaded to the lowest of the Plimsoll marks. When she arrives at her tropical freshwater port she will have sunk in the water to her highest mark.
Our very existence is intricately bound up with seawater, for without it we cannot live. Not only do we receive fresh water from it through solar distillation, via the atmosphere in the form of clouds from which rain falls to the earth, but bacteria and plankton living in the sea help create the air we breathe. The oceans’ food chains, including the seawater fish we eat, depend upon the growth of healthy phytoplankton. If we upset the balance of the oceans’ by polluting them, we threaten our very existence.
A swim in the sea can be refreshing to body and soul. We may read of the long-distance sailor who takes a swim when his yacht is becalmed upon a blue ocean; he plunges into the cool water and comes out feeling like an entirely new man, relaxed and yet invigorated. The very best sea swim I ever had was at a little cove at the Island of Formentera in the Mediterranean. There the water was wonderfully buoyant, because of its high salinity. I could float on my back with my arms and legs outstretched while I sunbathed too!
I am fascinated with water, particularly the way in which it can distort images through reflections and refractions, and I love watching waves braking on the seashore, whether on a soft sandy beach, a pebbly one, or hard granite rocks, as one finds in Cornwall. How wonderful it is to watch wriggling patterns of moving shadows on sand below the water’s surface, to see weeds sway at the behest of underwater currents, and waves dissipating on a beach. It’s no wonder children laugh and scream when they paddle in the amazing stuff.
Text for the Day
Revelation 21:1 ‘Now I saw a new heaven and a new earth, for the first heaven and the first earth had passed away. Also there was no more sea.’
Links
Seawater
http://en.wikipedia.org/wiki/Seawater
The Plimsoll Line
http://www.grandpapencil.net/science/plimsoll.htm
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