106 THE WATEES. 



away from the water at D, and leaving it a little behind, will 

 produce a high tide there, this tide is known as the " inferior 

 tide " (2). Again as the force at B is greater than at c, the 

 water at B will be pulled away from the earth and produce 

 a still higher tide, called the " superior tide " (1), or that 

 which takes place when the moon is due south. This heaping- 



DIAGRAM OF THE TIDES. ' 



up of the waters by the moon, to the height of 4-|- feet (on 

 an average) causes corresponding depressions to the same 

 depth in those parts of the earth which are situated between 

 the raised parts, making a difference between high and low- 

 water of nine feet due to the moon; precisely the same 

 effects are produced by the sun, but though his attractive 

 power is so much greater than the moon's, his distance is 

 also so much greater, that the heap of water he can raise is 

 somewhat less than 2 feet, and makes therefore less than 

 4 feet difference between high and low- water, due to the 

 sun. When the sun and moon are both on the same side, 

 or on opposite sides of the earth (as at new or full moon), 

 then the attraction of both act together and produce " spring 

 tides," with a difference of 13 feet ; at first and last quarter 

 the sun and moon oppose each other, acting in cross directions, 

 and the difference is then only 5 feet. But these effects 

 could only take place in the wide open space of an ocean, 

 and when other forces do not interfere, such as obstruction 

 to the tide- waves by land, &c. 



Inland seas (as large as the Mediterranean even) have 

 but very little tide, while pinched-up rivers may have a 

 great rush of tide, as at Bristol, where it rises to 45 feet 



