THE TIDES. (APP. E.) 225 



taken, as the rise and fall of the spheroidal surface 

 that would bound the water were there no dry land 

 (uncovered solid). To illustrate this statement, let 

 us imagine the ocean to consist of two circular 

 lakes, A and B, with their centres 90 asunder, on 

 the equator, communicating with one another by a 

 narrow channel. In the course of the lunar twelve 

 hours the level of lake A would rise and fall, and 

 that of lake B would simultaneously fall and rise to 

 maximum deviations from the mean level. If the 

 areas of the two lakes were equal, their tides would 

 be equal, and would amount in each to about 7/8 of 

 a foot above and below the mean level ; but not so 

 if the areas were unequal. Thus, if the diameter of 

 the greater be but a small part of the earth's qua- 

 drant, not more, let us say, than 20, the amounts 

 of the rise and fall in the two lakes will be inversely 

 as their areas to a close degree of approximation. 

 For instance, if the diameter of B be only i/io of 

 the diameter of A, the rise and fall in A will be 

 scarcely sensible ; while the level of B will rise and 

 fall by about if feet above and below its mean ; 

 just as the rise and fall of level in the open cistern 

 of an ordinary barometer is but small in comparison 

 with fall and rise in the tube. Or, if there be two 

 large lakes, A, A', at opposite extremities of an 

 equatorial diameter, two small ones, B, B', at two 

 ends of the equatorial diameter perpendicular to 

 that one, and two small lakes, C, C', at two ends of 

 the polar axis, the largest of these being, however, 

 VOL. in. Q 



