SALTNESS OF THE SEA. 105 



of sodium) which exists in the earth in great abundance in 

 layers of a crystalline structure, and as this chloride of 

 sodium (common salt) is soluble in water, of course it is all 

 dissolved by the water in whatever situation they may 

 come into contact with each other. The composition of sea- 

 water differs slightly in different parts of the earth, the 

 southern seas being slightly more salt than the northern. 

 As a general rule, about five per cent, of solid matter is con- 

 tained in sea- water, of this rather more than half (5*7) is 

 chloride of sodium ; the greater part of the other half consists 

 of different salts of magnesia, and this is the whole source of 

 the medicines known as Epsom salts and magnesia, the 

 former being the sulphate and the latter the carbonate of 

 magnesia. Iodine, another article used in medicine and 

 photography, is also extracted from sea- water, which how- 

 ever contains but a very minute portion, too small to be 

 detected as a general rule, but extracted by burning certain 

 sea-weeds, in the ashes of which it is found in sufficient 

 quantity to be separated. 



Although the sea as a whole keeps its level, yet in various 

 parts it is constantly rising and sinking, sometimes at one 

 place and sometimes at another ; these risings and fallings 

 are called tides, and are caused by the joint action of the 

 sun and moon, that is, by their attraction, acting at different 

 distances. First it must be observed (as is well known) 

 attraction varies inversely as the square of the distance, 

 that is, if the moon were twice as far off, her attraction of 

 the earth would be one-fourth of what it now is, if three 

 times as far off, one-ninth, and so on. 



The moon's average distance from the centre of the earth 

 (A to c, in the diagram) is about 240,000 miles, or about 60 

 times the earth's radius (B c), which is a little under 4000 

 miles, consequently if the distance from the moon to the 

 centre of the earth (A c) be expressed by 60, the distance to 

 the nearest surface (A B) will be 59, and the farthest surface 

 (A D) 61. The moon will attract the point B with a force 

 of -g 1 g-2 or 3 ^ ! , the point c with the force of -^ 2 or 36 1 00 , 

 and the point D with the force of -^ z or 3 7 \ 1 . Now as the 

 force at c is greater than at D, the earth itself will be pulled 



