Tlieories of the Tides. 223 



half its breadth * ; and if you really meant to account for the 

 rising of the tides by pressure, it is wonderful that this idea 

 never once entered your thoughts, as it would have saved you 

 the necessity of supposing, that a wave or tide could move at 

 the incredible rate of 500 miles an hour. But, at all events, it 

 is very easy to prove that, if this principle will not produce a 

 sensible rise of the tides, a greater breadth of ocean can do little 

 more than add another cypher to the sum of nothing. Let 

 ABC (fig. 2.) represent a quadrant of the earth's circumfe- 

 rence, and BC the boundaries of the Atlantic Ocean, which is 

 about 45° broad. Now, upon the principle of pressure and 

 equilibrium, if we suppose the moon to be vertical at A, there 

 would be a rise at B and a fall at C exactly proportioned to the 

 difference in their specific gravities ; but all the while the moon 

 was moving from A to B, which would take about three hours, 

 the waters at B would remain stationary at the same heio-ht, 

 because the specific gravities of the waters in the Atlantic 

 Ocean, during the whole of this time, would necessarily bear the 

 same proportion to each other, and consequently must produce 

 similar effects. In three hours after the moon had passed B, 

 she would be vertical at C, and then it would be hwh. water at 

 C and low water at B, and would continue so the next three 

 hours ; so tl»at, if the tides were produced by the pressure of the 

 waters within the Atlantic, they ought to remain stationary at 

 high and low water for the space of three hours without any- 

 apparent rise or fall ; and, as this phaenomenon has never yet 

 been observed, it is fair to conclude diat the tides are not jiro- 

 duced by pressure. 



Here, jierhaps, it will be argued, that if it be the nature of 

 fluids to press the lighter particles upwards luitil the equili- 

 brium be restored, there must necessarily be a rise and iiiU of 

 the waters produced by pressure, and that these effects ought 

 to be visible. To this I reply, that these effects are entirely 

 prevented by the moon's motion. Let A and B represent two 

 a<ljoining portions of water in the ocean, and suppose the spe- 



* If we suppose any part of tlie ocean to be 90° broad, twelve miles tlcep, 

 and the waters at one extremity to be twice as heavy as they are at the other, 

 there wonhl necessarily be a rise of four miles at one cnd'and a f:ill of four 

 miles at the other; so tiiat eij,dit miles depth of heavy particles nnght just 

 balance sixteen miles of those which had only half their weight. Now if we 

 suppose the waters in this ocean to retain the same sjjccinc gravitv, gradu- 

 ally increasing from one end to the other, hut that Iw/fxhv: breadth was cut 

 oil; the waters at one extremity would only be one-half part heavier than the 

 other extreme; and yet, if the ocean was twenty-four miles, or twice as 

 deep, there would be a fall at one end and a rise at the otl.er of more than 

 four milc^and three quarters, in order to restore the eqiiilil.'rium ; and con- 

 sequently twice the depth of ocean will more than comi)eiisate for the loss 

 of half Its brcadlh. 



cific 



