128 THE OCEAN. 



them. Ice-fields often acquire a rotatory motion; 

 and when we consider the immense weight 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 placed high and dry upon the ice! 

 From this cause arise many of the accidents which 

 give to the navigation of the Arctic sea its peculiarly- 

 hazardous character. 



When the temperature of the atmosphere is about 

 two or three degrees above the freezing-point, a 

 surface 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, Cap- 

 tain 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 composed, on its upper surface, 

 of numberless irregular, needle-like crystals, placed 

 vertically, and nearly close together; their length 

 varying, in diflferent 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 sur- 

 face of ice having this structure, sometimes looks 



