CIO TIDES. [Lesson xxxi. 



nadir, and which must be diametrically opposite to 

 the former : for in the hemisphere farthest from the 

 moon, the parts in the nadir being less attracted by 

 her than the other parts which are nearer to her, 

 gravitate less towards the earth's centre, and con- 

 sequently must be higher than the rest. Those 

 parts of the earth, on the contrary, where the moon 

 appears on the horizon, will have low water: for as 

 the waters in the zenith and nadir rise at the same 

 time, the waters in their neighbourhood will press 

 towards those places to maintain the equilibrium ; 

 to supply the place of these, others will move the 

 same way, and so on, to the places ninety degrees 

 distant from the zenith and nadir, where the water 

 will be the lowest. 



Let the diurnal rotation of the earth be now taken 

 into the consideration, and it is evident, that every 

 portion will pass twice through the elevated, and 

 twice through the depressed parts, so as to produce 

 two Tides in the day. But the places of high and 

 low water are now altered ; for the impressed 

 motions of rising and falling are retained for some 

 time after the forces which produce them are 

 greatest: and the greatest elevation happens about 

 three hours after the meridian of the place has 

 passed under the luminary, when it points about 

 half a quadrant to the east of it : and the water 

 continues to descend for 90 degrees hence, or till 

 the meridian points about half a quadrant to the 

 east of the next quarter. But in shallow seas, and 

 in the mouths of rivers, the Tides are retarded till 

 the fourth, fifth, or perhaps, sixth hour, after the 



meridian 



