Mr. E. L. Garbett on Popular Difficulties in Tide Theory. 179 



to 12 hours, and every "age of the tide/' from 1 or 2 hours 

 about Juan Fernandez to some 60 at Tide-end-town (Tedding- 

 ton), the earth-tides must everywhere have sensibly the same 

 establishment ; and this between and 6 hours, or almost cer- 

 tainly between 2 and 4. Between these limits also fall doubtless 

 the establishments of the great majority of oceanic surface, at 

 least within the tropics 12 . The main body, then, of the tides of the 

 globe's warmer half^ I regard as the mere difference between a 

 sea and an earth-tide, and hence their smallness. There seem 

 even large districts, quite pelagic, where the minimum of skin 

 or "surface viscosity" allows the earth -tide so nearly to equal 

 the marine as to leave the latter insensible, as at Tahiti, and also 

 the Virgin Isles, where I found^ at St. Thomas in 1845, not a 

 range of four inches during the day after a full moon. And 

 these are, as we might expect, the regions giving otherwise most 

 evidence of instability — in the latter case the evidence from fre- 

 quency of earthquakes being less striking than the numerous 

 records of shocks, in most of the Caribbean islands, immediately 

 following or accompanying their chief hurricanes; that is, as I 

 suppose, the mere removal, from a wide surface, of about a fif- 

 teenth of the accustomed pressure, or two barometric inches 

 (where at no other time is the range one-twentieth of an inch) 

 suffices to derange the equilibrium; and in the case of the St. 

 Thomas's hurricane of two years ago, it began a series of shocks 

 lasting months, after still longer quiescence. 



Now, on looking through Homme's Tables, we find all the 

 intertropical cases of tides approaching an average magnitude 

 have establishments of under 2 or over 4 hours ; all that can be 

 called large have them between 6 and 12; and all the largest 

 between 8 and 10, in other words, the water- tide so modified 

 from a primitive one as to make high water arrive with low earth 

 (if I am right), and low water with high earth. The greatest 

 lengths of coast having either such establishments, or tides ap- 

 proaching ours, are confined to three bays (of Bengal, Guinea, and 

 Panama), and the correspondence of the two elements is most stri- 

 king. The nearer the establishment to 9 hours, as about Panama, 



la Romme, Tableau des Vents, des Marees, &c. i. p. 388. According to 

 Airy, ' Tides and Waves,' art. 281, 282, " Tf the period of the forced tide- 

 wave be less than that of a free wave . . . (i. e. if the wave be urged along 

 more rapidly than it would go alone) ... it is low water under the moon ;" 

 and in the contrary case, " (i. e. if the water were so immensely deep that 

 the wave would travel alone more rapidly than the disturbing forces urge 

 it along) . . . there would be high water under the moon." Now as the 

 former is the case of our deepest oceans, but the latter that of the internal 

 fluid abyss, or earth itself, it would follow that, were there no friction, the 

 general water establishment would be 6 hours, and the ground's establish- 

 ment or 12. The general effect of friction is to make the hour of high 

 water earlier, but of high earth later. 



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