48 



THE POLAR WORLD. 



FORMS OF ICE15EKGS. 



which may possibly be revolving with equal rapidity in an opposite direction 

 when masses not seldom twenty or thirty miles in diameter, and each weigh- 

 ing many millions of tons, clash to- 

 gether, imagination can hardly con- 

 ceive a more appalling scene. The 

 whalers at all times require unremit- 

 ting vigilance to secure their safety, 

 but scarcely in any situation so much 

 as when navigating amidst these 

 fields, which are more particularly 

 dangerous in foggy weather, as their 

 motions can not then be distinctly ob- 

 served. No wonder that since the 

 establishment of the fishery numbers 

 of vessels have been crushed to pieces 

 between two fields in motion, for the 

 strongest ship ever built must needs 

 be utterly unable to resist their power. Some have been uplifted and thrown 

 tipon the ice ; some have had their hulls completely torn open ; and others 

 have been overrun by the ice, and buried beneath the fragments piled 'upon 

 their wreck. 



The icebergs, which, as their name 

 indicates, rise above the water to a 

 much more considerable height than 

 the ice-fields, have a very different or- 

 igin, as they are not formed in the 

 sea itself, but by the glaciers of the 

 northern highlands. As our rivers 

 are continually pouring their streams 

 into the ocean, so many of the gla- 

 ciers or ice-rivers of the Arctic zone, 

 descending to the water-edge, are 

 slowly bxit constantly forcing them- 

 selves farther and farther into the 

 sea. In the summer season, when 

 the ice is particularly fragile, the 

 force of cohesion is often overcome 

 by the weight of the prodigious mass- 

 es that overhang the sea or have been 

 undermined by its waters ; and in the 

 winter, when the air is probably 40 

 or 50 below zero and the sea from 



28 to 30 above, the unequal expan- FOKMS OF ICEBERGS< 



sion of those parts of the mass ex- 

 posed to so great a difference of temperature can not fail to produce the sep- 

 aration of large portions. 



