MOMENTUM OF TIDAL CURRENTS. 133 



earth's currents on the moon renders it perma- 

 nently oval, or egg-shaped, as discovered by 

 improved telescopes ; because that globe has not 

 a rapid axial rotation like the earth, to vary the 

 attractive force. 



Although the average height of the rise of the 

 tides is only three or four feet, yet in some localities 

 they rise thirty or forty feet. This extraordinary 

 elevation is due to the momentum of tidal currents, 

 whereby vast masses of ocean-waters are put in 

 motion, impinging against shelving shores and 

 narrow bays, and force up the water, as by cur- 

 rents in hydraulic rams, to considerable heights. 



A little additional rise of the tides is produced 

 by the centrifugal tendency of the ocean-waters 

 to recede from the centre of the earth, by its 

 monthly orbital revolution about the common 

 centre of the moon and earth, while they swing 

 around each other. 1 



THE RELATIVE EQUATORIAL POSITIONS OF THE MOON 

 AND EARTH SUSTAINED BY THE DIAMAGNETIC CIRCU- 

 LATION OF ELECTRIC CURRENTS. 



The northern and southern declinations of the 

 moon, denoted its librations, affect magnetic nee- 

 dles, as stated by Professor Bache. This action, 



1 The similar centrifugal tendency induced by the daily rotation of the 

 earth bulges out the equatorial region to thirty miles greater diameter 

 than the axial measure ; making a rise of nearly twelve feet to the mile 

 in proceeding from the pole to the equator. The Mississippi River, in 

 flowing from north to south, recedes from the centre of the earth eight 

 or nine feet to the mile, and flows by the centrifugal tendency of its 

 waters. 



