THE NON-LIVING WORLD 183 



Thus it is (i) when the sun and moon are exactly opposite 

 each other, the world being between them (which is 

 the time of full moon), and (2) when they are on the same 

 side, or in conjunction, the world being opposite both 

 (which is the time of new moon), that their combined 

 actions produce the very high tides. The intermediate 

 periods give rise to the relatively slight, or "neap," 

 tides. 



The ellipticity of the earth's orbit and also that of 

 the moon, cause variations in the proximity of these 

 bodies to each other and to the sun. It is when the sun 

 and moon are thus nearest to us, that we get the very 

 high or " spring " tides. 



The various configurations of tracts of land and water, 

 and their geographical positions, variously modify the 

 times and degrees of elevation in the tides of different 

 places. Thus, for example, the Mediterranean is an 

 almost tideless sea. 



An action, which is so visible with respect to the 

 ocean, must also take place in the atmosphere, and thus, 

 as before mentioned,* there must be atmospheric tides, 

 though they do not make their existence conspicuous. 



The phenomena known as "falling stars" are due to 

 the attraction to the earth of minute cosmical bodies, 

 meteorites, which it encounters in its path round the 

 suu. These bodies afford us the plainest proof that the 

 same chemical substances exist in the solar system, 

 external to this earth, as exist in the earth itself. But 

 the careful study of the spectrum spectrum analysis 

 which can be obtained from the light of the stars, tends 

 to show us that a similar identity of materials exists 

 between the substances which compose our own planet and 



* See ante, p. 153. 



