68 The Realm of A T ature CHAP. 



of which are on opposite sides of the Earth, and equal troughs 

 between them. As the Earth rotates, high water and low water 

 succeed each other regularly, from east to west, as the crest 

 and trough of the wave pass at intervals of about 6|- hours. 

 Without mathematical reasoning it is impossible to explain 

 how the tidal wave, pulsating round the world, is related to 

 the actual position of the Moon in its orbit and in the sky 

 ( 218). On account of the formation of tidal currents, the 

 hydrosphere is very gently pressed like a brake on the 

 lithosphere by the differential attraction of the Moon ; and as 

 the energy of the currents comes from the Earth's rotation, 

 the rate of rotation at the end of each century is slower by 

 the fraction of a second, and the time of rotation, or day, is 

 longer in the same minute proportion. 



104. The Tidal Romance of the Moon. Millions of 

 years ago the Earth must have rotated much more rapidly 

 than now, when it suffers from long application of the brake. 

 At that remote epoch the Moon was much nearer than 

 now, for it is a property of revolving bodies, which cannot 

 be explained here, that any reduction in the rate of the 

 Earth's rotation is necessarily accompanied by an increase 

 in the Moon's distance. The nearer Moon must have raised 

 far greater tides than those we now know, in the more 

 extensive and denser hydrosphere of those ancient days. 

 In the remotest past on which this argument casts light 

 the Moon must have been close to the Earth, whirling 

 round its little orbit in the same time as the Earth spun 

 round on its axis, which was then only a few hours. The 

 Moon, indeed, seems to have been originally part of the 

 semi-fluid Earth whirled off by the furious rotation (93) 

 of the earliest times. As the Moon receded from the Earth 

 in its slowly widening spiral path it also had a hydrosphere 

 in which the Earth's differential attraction raised tides, the 

 friction of which gradually brought the rapid rotation of our 

 satellite to correspond with its period of revolution round 

 the Earth. 



105. The Sun even more conspicuously than the Moon, 

 separates itself from the other heavenly bodies, which are 

 dim by contrast with its brilliance, and when the Sun 



