342 THE VOYAGE OF THE 'DISCOVERY' [Appx. 



high latitudes the sea freezes over during the winter months often 

 in a very uniform sheet. This sheet breaks up during the summer 

 and floats north to form the pack-ice, which is usually encountered 

 by exploring ships near the Antarctic Circle. The pack varies from 

 year to year, and its structure in Arctic regions has been described 

 by Dr. Drygalski and others. The first two inches frozen are 

 composed of plates of ice lying horizontally. These plates are 

 usually under half an inch across and are separated from the 

 greater mass of ice by a half-inch layer of very confused crystals. 

 The major part consists of bundles or sheaves of fibres arranged 

 perpendicularly to the surface of freezing, and this fibrous structure 

 extends to the bottom of any given floe. 



The sali7iity of sea-ice seems to depend more upon the rate of 

 freezing than upon the depth or distance from the upper surface. 

 The amount of salt \aries greatly, as may be seen if reference be 

 made to our observations on the salinity of the sea-ice. The mean 

 salinity is about 4-3 grammes of sodium chloride per Htre of melted 

 ice, but by fractional crystallisation a salinity up to 266-6 grammes 

 per litre may be produced. 



Snow accumulates on the surface of the ice, and by pressing it 

 down below sea level weakens it ; and sometimes so much snow 

 accumulates that a drift is formed which has its under surface 

 dissolved by the sea during the ensuing summer. Thus, then, we 

 see that it is possible for water substance to pass from the 

 atmosphere back to the ocean without taking part in the glaciation 

 of the land. 



Hummocks are rather exceptional in the sea-ice of McMurdo 

 Sound, but where the Ross ice-sheet moves in towards Winter 

 Quarters Peninsula a great series of hummocks was produced 

 some two miles long. These hummocks in the year 1903 rose to 

 heights of from twelve to twenty feet above sea level, and were 

 sometimes merely bucklings of the ice, or at other times were 

 fractured slabs standing up vertically. 



The ihick?iess of the sea-ice varies according to circumstances. 

 Where snow accumulates any thickness may be produced, but in 

 an exposed spot, such as 100 yards off Hut Point, the thickness 

 produced by freezing is under 8^ feet in the year. In Arrival Bay, 

 where the land precludes a rapid circulation of the water, as much 

 as ten feet has been produced by freezing alone. The following 

 observations were made at the ice-gauge oft" Hut Point : 



