GEOGRAPHIC LITERATURE 365 



with an arrow diagram, which is surely the simplest possible method 

 of showing the equality of the centrifugal forces. The diagram and ex- 

 planation of horizontal components of the tide-raising forces is equally 

 simple and lucid. For the dynamic theory motion in the masses sub- 

 jected to periodic impulses is the feature of actuality not contemplated 

 in the equilibrium theory. 



In a continuous equatorial canal of some 13 miles' depth we should 

 have free oscillations that would pass around the earth with the moon. 

 In less depths, and our oceans are much shallower, the wave would go 

 slower. Thus the moon's periodic impulse is quicker than the free oscil- 

 lation, and the resultant oscillation is inverted with low water always 

 under the moon. About such a state of affairs occurs in the Pacific, and 

 it is completely opposed to the equilibrium view. The fact that the 

 Pacific is not an equatorial canal, however, forbids us to account this 

 theorem as more than a suggestion. For regions where the tide follows 

 the moon by irregular intervals Professor Darwin seems to fall back on 

 Whewell's abandoned cotidals. Here we are beyond the grasp of mathe- 

 matical treatment. Tides in canals or on a uniform ocean-covered globe 

 admit of analysis, but the actual geographic problem has not been solved ; 

 even a large lake is of doubtful solution (p. 185). Ferrel declares his 

 conviction that an equatorial dike across the Atlantic, though barring 

 out all waves from the southern ocean, would not alter the actual tides 

 Of the North Atlantic. Darwin seems to consider the wave from the 

 south as more significant than the local tide. " It may be conjectured 

 that on the coast of Europe the latter is of less importance than the 

 former" (p. 188). The whole subject is full of conjectures at this point. 

 "The problem is one of insuperable complexity " (p. 188). Dr AVhewell 

 was obliged to abandon his famous chart of cotidals in 1836 on two 

 grounds: (1) the excessive contouring of well-determined cotidals, and 

 (2) the great difference of epoch of the diurnal wave in Europe and Amer- 

 ica, together with the identical epoch in Spain and at the Cape of (Jood 

 Hope, supposed to be separated by a long journey up the Atlantic. No 

 answer has ever been made to this objection, yet Professor Darwin again 

 appeals to this cotidal chart abandoned by its author in the second year 

 of its age, since copied in innumerable hand-books, and made responsible 

 for the mythic birth of the tide in the Pacific. 



One fancies that the author found the closing chapters, dealing with 

 the role of tidal friction in the universe, most pleasant to write. Here 

 Professor Darwin is peculiarly in his own domain, and his exposition is 

 of the happiest. Looking back to days when the earth was still a glow- 

 ing, fluid mass, we are made to see its molten tides rising toward the 

 moon and struggling against the friction of particle on particle. In this 

 way somewhat delayed, flood height is only reached when the earth's 

 rotation has carried it somewhat forward past the moon. This high-tide 

 protuberance pulls the moon forward in its orbit, which is thus enlarged 

 and the month lengthened. At the same time the moon, striving to 

 keep the tide crest under her, resists the earth's rotation and prolongs 

 our day. Longer and longer grow both day and month, though at un- 

 equal rates, and must do so, even under the slighter impulse of the present 



