THE EARTH. 



the rocks. It does not melt on the upper surface, 

 but underneath ; and often cracks into many 

 larger or smaller clefts, from whence the thawed 

 water trickles out. By this it becomes, at last, so 

 weak, that being overloaded with its own ponder- 

 ous bulk, it breaks loose, and tumbles down the 

 rocks with a terrible crash. Where it happens to 

 overhang a precipice on the shore, it plunges into 

 the deep with a shock like thunder ; and with 

 such an agitation of the water, as will overset a 

 boat at some distance, as many a poor Green- 

 lander has fatally experienced." Thus are these 

 amazing ice mountains launched forth to sea, and 

 found floating in the waters round both the Poles. 

 It is these that have hindered mariners from dis- 

 covering the extensive countries that lie round 

 the South Pole ; and that probably block up the 

 passage to China by the north. 



I will conclude this chapter with one effect 

 more produced by the saltness of the sea ; which 

 is, the luminous appearance of its waves in the 

 night. All who have been spectators of a sea by 

 night, a little ruffled with winds, seldom fail of 

 observing its fiery brightness. In some places it 

 shines as far the eye can reach ; * at other times, 

 only when the waves boom against the side of the 

 vessel, or the oar dashes into the water. Some 

 seas shine often ; others more seldom : some, ever 

 when particular winds blow ; and others, within 

 a narrow compass ; a long tract of light being 

 seen along the surface, whilst all the rest is hid in 



* Boyle, vol. i. p. 294. 



