DISCOVERY 



263 



only a small fraction of the total surface of separation, 

 and breaking constantly occurs along old pressure- 

 ridges. 



The heaviest pressure-ridges are formed b}' a some- 

 what different process. Suppose a crack has divided 

 a heavy floe into two pieces, each perhaps several miles 

 square, and that they then move together again. They 

 will rarely come together so that they fit exactly, for 

 there will nearly always be some lateral displacement. 

 Most of the pressure between the two will be taken 

 on a few projecting corners. In addition there may 



the Endurance, became more common towards the 

 end of the winter. It did not as a rule occur during 

 gales, but generally in periods of calm, often shortly 

 before a gale. We may perhaps get an idea of its 

 causes from the following analogy. 



Wind Effects on Drifting Pack-ice 



Suppose a jig-saw puzzle, emptied out on a smooth 

 table, is pushed along by means of a ruler. This ma}' 

 be taken as representing pack-ice drifting under the 

 influence of a wind. The analogy is not quite accurate. 



Fig. 2.— .SH.\CKI,ET0X A^O WH.D STANDING BETWEEN HUMMOCKS OF ICE. 

 By cctrteiy of WiUiam Heinemaiin, 



also be a twisting or screwing motion of the ice. If 

 the pressure goes on, a buckling of the floes at the 

 regions of contact wiU occur. Blocks from one floe 

 wUI be driven up over the surface of the other, and 

 in this way ridges are formed of blocks perhaps 5 or 

 6 ft. thick. This process is known as " rafting." 

 Pressure-ridges seldom reach a great height. Thirty 

 feet was very e.xceptional in the Weddell Sea, and 

 from 10 to 15 ft. above the water-line may perhaps be 

 taken as an average height. The formation of pres- 

 sure-ridges by screwing and rafting is the greatest 

 danger to which a ship drifting in the pack-ice is 

 exposed. This type of pressure, during the drift of 



since the wind, when conditions are steady, will act 

 all over the surface of the ice, and not just along the 

 edge of the field ; but if we suppose all the pieces of 

 the puzzle to be moving steadily to begin with, this 

 will not matter. Now suppose we move the ruler and 

 apply it in a different direction. This represents a 

 change in the direction of the wind, not taking place 

 simultaneously all over the ice-field, but beginning at 

 one edge of it. After a time all the pieces of the puzzle 

 will get into steady motion in the new direction, but 

 during the change from one motion to the other much 

 relative movement of the pieces will take place. They 

 will rub against one another, and twist round, in fact 



