POLYPES AND CIRCULATION. 101 



from the sea-water by the act of myriads of 

 polypes at the same instant, must lessen the 

 specific gravity of the water with which they are 

 in contact, and from which they extract one of its 

 constituent parts. Whatever the actual weight of 

 the lime thus secreted may be, and it may in the 

 construction of one reef alone amount to thousands 

 of pounds in a day, ^- that weight is so much 

 abstracted from the wafer, which being thus lighter 

 than the strata of water over it rises upwards to 

 the surface, and is replaced by water heavier, 

 salter, and charged with the lime required by the 

 little reef builder, for the work which he could 

 not carry on if the water he had deprived of its 

 lime remained around him without being replaced 

 by a new supply. Thus the marine insect may 

 have a very important office to perform in the 

 circulation of the ocean waters, and the function 

 the polype thus discharges is not accidental. It 

 is exercised by the design of Him who gave the 

 creature its power to secrete the lime and the 

 instinct with which to labour. 



The waters which on becoming heavier, because 

 rendered salter, by evaporation, sink do wnward, 

 are likewise warmer than those which ascend to 

 supply their place. Thus the circulation of the 

 sea modifies not only its own temperature, but 

 the temperature of the climates of the lands it 

 washes. This effect is produced on a great scale in 

 some parts of the globe ; and an illustration of it 

 is presented by our own shores. The western 

 shores of the British islands, and especially the 



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