e-REENLAND OR POLAR ICE. 



265 



great many pieces, few of which will exceed forty 

 or fifty yards in diameter, Now, such a number 

 of these pieces collected together in close contact, 

 so that they cannot, from the top of the ship's 

 mast, be seen over, are termed 2l pack. 



When the collection of pieces can be seen across, 

 if it assume a circular or polygonal form, the name 

 of patch is applied, and it is called a stream when 

 its shape is more of an oblong, how narrow soever 

 it may be, provided the continuity of the pieces 

 is preserved. 



Pieces of very large dimensions, but smaller 

 than fields, are called floes : thus, a field may be 

 compared to a pack, and a floe to a patch, as re- 

 gards their size and external form. 



Small pieces which break off, and are separated 

 from the larger masses by the effect of attrition, 

 are called brash-ice, and may be collected into 

 streams or patches. 



Ice is said to be loose or open, when the pieces 

 are so far separated as to allow a ship to sail free- 

 ly amongst them ; this has likewise been called 

 drift-ice, 



A hummock is a protuberance raised upon 

 any plane of ice above the common level. It 

 is frequently produced by pressure, where one 

 piece is squeezed upon another, often set up- 

 on its edge, and in that position cemented by 

 the frost. Hummocks are likewise formed, by 

 pieces of ice mutually crushing each other, the 



53 



