THE AUTHOR 's THEORY OF THE TIDES 173 



member, not of practical astronomy; for of the latter I 

 have no words but of praise. We have just seen how, in 

 the case of cometary tails, astronomers have invented 

 light repulsion to take the place of the principle of equi- 

 librium, which they wrongfully discarded, and we may 

 now expect them to settle upon some equally grotesque 

 theory to account for the present phenomenon so as not 

 to disturb Newton's irrational explanation of precession. 

 Lord Kelvin, Newcomb, and others have sought to ex- 

 plain the mystery on the basis of Newton's tidal theory 

 by first deducing the coefficient of the earth's elasticity 

 and employing this as a new basis on which to found 

 further deductions. Others try to connect the anomaly 

 up with continental risings and sinkings, with earth- 

 quake cycles, with atmospheric shiftings, with the alter- 

 nating deposition and melting of snow and ice, and what- 

 not. 



Paradoxical as it may sound to the ears of astrono- 

 mers, it is NOT TRUE that the earth's axis shifts within her 

 crust; nor is it true, as they imagine, that the cycles de- 

 scribed are basically due to long-period gyrations. On 

 the contrary, the phenomenon is produced by diurnal 

 oscillations of the planet as a whole and the annual and 

 430-day periods are simply periodical changes in the am- 

 plitudes of those diurnal movements. An illustration 

 may help to make the matter clearer: You remember 

 how, in Foucault's celebrated pendulum experiment (by 

 which he proved the rotation of the earth), the tip of the 

 pendulum was made, at the end of each swing, to cut 

 into a little circular ridge of sand. At the close of the 

 experiment the pendulum tip left its tiny dents all round 

 the circle as the only record of the oscillations, so that a 

 stranger happening upon the scene after the pendulum 

 had been removed might easily have supposed that the 

 marks were caused by a toothed wheel rolled round the 

 ridge, or in any way, indeed, save the right one. 



Now, astronomical observations, especially in cases 

 of such super-delicacy as required in this case, cannot be 

 made at any hour of the day or night, but only at the most 

 propitious time of all midnight. That is to say, the 



