THE ARCTIC SEAS. 115 



them. Ice-fields often acquire a rotatory motion ; 

 and when we consider the immense weiglit of these 

 ponderous masses, we shall have an idea of the 

 irresistible impetus communicated by such a body in 

 motion. Scoresby calculates one mentioned by him 

 at ten thousand millions of tons : no wonder that, 

 coming in contact with a vessel, her iron knees and 

 oaken timbers should be crushed like a walnut, or 

 that she should be lifted clean out of the water by 

 the pressure, and jjlaced high and dry upon the ice ! 

 From this cause arise manv of the accidents which 

 give to the na^dgation of the Arctic sea its peculiarly 

 hazardous character. 



AMien the temperature of the atmosphere is about 

 two or three degrees above the freezing-point, a sur- 

 face of ice, if placed in a horizontal plane, will melt, 

 not by a general dissolution of its substance, but so 

 as to leave a multitude of perpendicular columns, or 

 needles. In the late attempt to reach the North 

 Pole by boats hauled over the ice. Captain Parry 

 found ice in this condition productive of no little 

 inconvenience. At the very commencement of the 

 journey we find it thus noticed : "June 26. — A great 

 deal of the ice over which we passed to-day presented 

 a very curious appearance and structure, being com- 

 posed, on its upper surface, of numberless irregular, 

 needle-like crystals, placed vertically, and nearly 

 close together ; their length varying, in differeni; 

 pieces of ice, from five to ten inches, and their 

 breadth in the middle about half an inch, but pointed 

 at both ends. The upper surface of ice having this 

 structure, sometimes looks like greenish velvet ; a 



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