182 ELEMENTS OF SCIENCE 



of eccentricity (i.e., its greatest departure from a circular 

 path) is greatest, the pole which when thus most distant 

 from the sun, is also inclined away from it, must then 

 endure a very exceptional degree of cold. Of course, at 

 such a time, the opposite pole and hemisphere must be 

 very exceptionally warm. 



The direction also of the earth's axis slightly varies, 

 each pole describing a circle (comparable with that 

 described by the summit of a revolving teetotum) in a 

 very long period of time; that is to say, in nearly 

 twenty-six thousand years. This movement is spoken of 

 as the precession of the equinoxes, because each change in 

 the position of the earth's axis necessarily changes the 

 position, in the earth's orbit, of the spot where equal day 

 and night are experienced all over the globe. 



Evidently from the great proximity of the moon to 

 the earth, there must be an energetic action of gravity 

 between them, and this energy must produce its most 

 conspicuous effect upon what is at once most easily 

 moved and can most plainly be seen to be so moved. 



That which is thus most easily and evidently moved, is 

 the earth's liquid investment the ocean. The effect of 

 the action of gravity between the earth and the moon, is 

 made manifest by the tides the moon raising up the 

 surface of the sea as it revolves round the earth. This 

 action is, of course, modified by that of the sun, but on 

 account of the enormous distance of the latter, its action 

 is much less than that of our satellite. Did the moon 

 act alone, and were the earth perfectly spherical and 

 everywhere covered with a uniform depth of water, the 

 moon would so attract the water that there would be 

 one great wave, or heap of water, directly beneath it, 

 and another on the opposite side of the globe, the water 

 in the interspace being correspondingly depressed. 



