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 different 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 
