THE SPECIPIC GRAVITY OF THE SE^V, ETC. 211 



.the strength of fibre in the flower-stalks of plants (§ 303), but how 

 much more exquisite is the system of counterpoises and adjust- 

 ments here presented betv/een the sea and its salts, the wdnds and 

 the heat of the smi ! The capacity of the sun to warm, of the 

 sea water to expand, the quantity of salts these contain, and the 

 power of the "wind to suck up vapom% are all in such nice adjust- 

 ment the one with the other, that there is the most perfect com- 

 pensation. By it they make music in the sea, and the harmony 

 that comes peahng thence, though not of so lofty a strain, is never- 

 theless, like the songs of the stars, divine. 



439. Suppose there were no winds to suck up fresh water from 

 A thermal tide, the bruic of the occau ; that its average dej)th were 

 3000 fathoms; that the solar ray were endowed with power to 

 penetrate with its heat from the top to the bottom ; and that, from 

 bottom to top, the seas of each hemisphere, in thermal alternation 

 with the seasons, were raised to summer heat and lowered to mn- 

 ter temperatm^e : the change of sea level from summer to winter, 

 and from winter to simimer, in one hemisphere, would, from this 

 cause alone, be upwards of 125 feet ; and in its rise and fall we 

 should have, from pole to pole, the ebb and flow of a great ther- 

 mal tide that would tm-n with the sun in the ecliptic, and tell the 

 equinoxes by the march on the tide staff of its rising and falling 

 waters. But difference of level would not be all that w^ould give 

 strength and volmne to this tide ; difference of specific gravity 

 would lend its weight as so much dynamical force, which differ- 

 ence would create an upper and under annual tide from one 

 hemisphere to the other. This double distm'bance of equilibrium 

 would not give rise to a tidal wave — not mere motion without trans- 

 lation — but to a tidal flow and reflow of water from one hemi- 

 sphere to the other in volumes of vast magnitude, power, and 

 majesty. This is an exaggerated view of the dynamical force of 

 the simbeam ; but it is presented to show the origin of the ther- 

 mal tide shown on Plate lY. The difference between the actual 

 and the supposed thermal tides is one of degTce merely ; for the 

 sea water that is hable to any considerable change of temperature, 

 instead of reaching from the bottom to the top, is scarcely more 

 than a " pelhcle " to the ocean. Nevertheless, there is a regular pe- 

 riodical flow and reflow between the poles and the equator. It is 

 the annual ebb of this tide which fills the upper half of the North 

 Atlantic with icebergs every spring and summer. The heated 

 portion forms a stratum or layer which is thickest at the equator, 



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