VI.] THE MOTIVE POWERS OF THE UNIVERSE. 07 



whatever; it is that other portion of the unexhausted 

 energy of the original nebula which exists as planetary 

 motion. So that whatever work is done by the tide (as 

 for instance if it works a water-mill)/ the energy trans- 

 formed in doing that work is a part of the energy of motion 

 due to the earth's rotation ; and, by the friction of the tidal 

 wave against the earth, the velocity of the earth's rotation 

 is being constantly lessened, though by an immeasurably 

 small quantity. 



We thus come to the wonderful but highly probable Energy of 

 conclusion, that the motive power of a tidal current and tide ; their 



that of a wind-driven current liave both of them the "^oi^^o^ 



source. 



same origin, in the potential energy due to the gravi- 

 tation of the parts of the original nebula on each other. 

 Part of this potential energy was transformed into the 

 energy of motion due to tlie earth's rotation, and in that 

 form became the motive power of the tidal current. 

 Another part was transformed into the energy of motion 

 of meteoric masses, and this into solar heat ; which, after 

 it reached the earth, was transformed back again into the 

 motion of the winds and currents. 



' Tyndall on Heat as a Mode of Motion, p. 425. 



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