176 Mr. E. L. G-arbett on Popular Difficulties in Tide Theory. 



dered to have verified) that this cause might have counteracted 

 and masked the shortening due to thermal contraction, so as to 

 account for the non-diminution (or, as now admitted, lengthening) 

 of the day since Hipparchus's time. I did not say, as Mayer is 

 represented to have done, that tides would "stop the earth's ro- 

 tation " 3 , but only retard it ; and of what either he or his prede- 

 cessor Kant really said on the matter I was and remain quite 

 ignorant 4 . As an obvious consequence of the established view of 

 tide-causation, I supposed it common property, — the loss of vis 

 viva seeming to be a plain truism, from the mere fact that corn has 

 been ground by tide-mills, the power spent having no possible 

 source but the earth's original rotary spin. 



The chief difficulty, however, about tide theory has always 

 struck me as lying in its involving tides obviously the greatest 

 at the equator, where in fact they are smallest on the whole. 

 The exceptionally high tides of the British seas, which seem 

 equalled only in a few areas yet smaller, lead us to form far too 

 high an estimate of the phenomenon at large, and to doubt 

 whether Newton's deductions as to the average lunar and solar 

 effects (say 7 and 2 feet) suffice for the known phenomena ; when 

 in fact they far more than suffice. Remembering these school 

 common-places, and going from our high-tided shores to the 

 Caribbean Sea, no part of which has tides exceeding, or barely 

 exceeding, a foot, and further, learning from navigators that 

 they regarded such barely sensible tides as general to nearly all 

 the tropical zone, especially its Pacific half, I was puzzled to 

 know what becomes of the theoretic neap waves of 5 or 6 feet, 

 and springs of 9 or 10. In their proper nursing-ground, between 

 the tropics, or at least the lunar tropics (which enclose, let us 

 remember, as nearly as we can know, just half the unfrozen earth 5 , 

 and probably more than half its watery surface), any approach 

 even to this theoretic average seems, by Romme's Tables, to be 

 known only in a few local river bores ; while all the cases deci- 

 dedly exceeding it, fluvial or marine, are in high latitudes — our 

 whole British plateau being such an exception, perhaps the most 

 extended of any, but as trilling on the earth's watery surface as 

 the two extreme areas of 40 feet tide, in the Severn and near 

 St. Malo, are upon itself. On the other hand, vast extents of the 

 tropical Pacific must, as appears from the case of islands like the 

 Tahiti group (said to have " always high water at noon, and low 

 at midnight"), be sensibly tideless — all lunar effect being masked 

 by the mere heaping-up and withdrawal of coast-water by day 



3 Phil. Mag. May 1863, p. 377- 



4 Phil. Mag. April 1866, p. 322, and June, p. 533. 



5 2 sin 28° 44 '(being moon's greatest declination) = sin 74° (the limiting 

 latitudes of unfrozen sea). 



