6l8 SCIENTIFIC RECREATIONS. 



of the sea, and the land would be thereby buried. That the sea does 

 encroach upon the earth we know, but it also recedes. Here is the balance 

 of Nature. 



Rivers pour in water and material. The sun absorbs the water and 

 prevents overflow ; tiny animals make shells from the material. All the 

 causes we have mentioned tend to permit the encroachment of the waters, 

 but volcanic action and even earthquakes act also to neutralize this tendency 

 by upheaving hills and mountains, which prevent the invasion of the sea by 

 its elevation or by land depression. We have seen in our chapters upon 

 GEOLOGY how the ocean beds have been upheaved, and remains of marine 

 animals are daily found upon our highest hills. Thus the forces which 

 sometimes cause such destruction in the earth are the means whereby the 

 waters are kept in their places. But for volcanic action the land might all 

 disappear by denudation and continual wear and tear, and be deposited 

 at the bottom of the sea ! 



If it were not for currents, of which many defined ones exist in the 

 ocean, and the never-ceasing flow and ebb of the tides, the sea would soon 

 lose its purity and clearness. Though the water is salt and becoming salter, 

 animalculse and all kinds of plant-animals would still increase and multiply; 

 so the decay of animal and vegetable matter would quickly render the 

 ocean a source of pestilence and death to mankind, and be most injurious to 

 animal life generally. But the movement is so ceaseless, and the various 

 fish and mammalia (whales, for instance), by preying upon each other, as 

 other animals on earth do, keep up the balance of production, and the organic 

 matter deposited in the sea is also cleared away. 



That the constant currents of the sea prevent the formation and growth 

 of seaweed is clearly shown by the great " Sargasso Sea," or tract of weed 

 (Fucus natans), called the Gulf-weed. This great tract embraces thousands 

 of square miles, and is situated in the very middle of the Atlantic Ocean, 

 where there are but few currents ; but surrounding it is the Gulf-Stream, an 

 enormous current of water running at a regular rate of four or five miles an 

 hour. This Gulf-Stream is supposed to be caused by the same laws and 

 influences which determine the trade-winds namely, a constant rarefaction 

 of the water at the tropical parts of the earth, and a corresponding conden- 

 sation at the Arctic portions, for warm water is much lighter than cold, and 

 when the waters of the tropical regions become lighter, the heavier waters 

 of tne cold regions pressing down more forcibly tend to raise them above 

 their proper level ; they therefore flow towards those very parts which have 

 sunk down by their contraction, and a constant current takes place ; this 

 current is the Gulf-Stream. It runs from the Gulf of Mexico northwards 

 towards Newfoundland, turning by Iceland towards the British Isles, by 

 France and Spain, onwards to the coasts of Africa and South America, the 

 West Indies, and again to the Gulf of Mexico, although the return current 

 does not go by the name of Gulf-Stream. This great stream of water, 

 warmed by the tropical sun, serves the same two purposes fulfilled by the 



