CURRENT 305 



depths in which bergs ground (120 to 150 feet bergs do not 

 touch bottom in more than 100 to 120 fathoms of water) ) 

 secondly, on an eye estimate of the proportion as indicated in 

 an overturned berg; and, thirdly, on the nature of the ice 

 itself as exposed in the face of the berg on the cliff from which 

 it has come. The transition from snow to ice is very gradual, 

 and strongly impresses one that the mass throughout must 

 contain large quantities of air — an impression supported by the 

 examination of some ice taken from the bottom of an over- 

 turned berg. It would be difficult, if not impossible, to 

 ascertain exactly this proportion by actual measurement, but 

 for the reasons which I have adduced I believe that it is not 

 greater than 5 to 1. 



I regard this factor as highly important. It would be of 

 interest if it only enabled the mass of these great floating 

 islands to be calculated, but it does much more ; for it must 

 be remembered that the bergs are detached from ice-sheets 

 covering or connected with the land, and that the dimensions 

 of the bergs give the thickness of the sheets from which they 

 have come. In fact, from these data we can make an 

 extremely interesting statement. If the average height of 

 bergs be taken as 150 feet, and the proportion of the sub- 

 merged to the visible part be accepted as 5 to 1, we can say 

 that 900 feet is the average thickness of the ice-sheet extending 

 over a very large area of the Antarctic Regions. 



Current. — The general drift of the current in the Ross Sea 

 is indicated by the direction taken by the bergs, to which I 

 have referred. It will be remembered that on our return 

 along the Barrier we had the good fortune to recognise a berg 

 which we had seen on our outward journey. It had drifted 

 seventy miles to the west in twelve days, but the surface water 

 had been moving at a greater speed, as we could tell by its 

 effect on the ship. The tidal streams do little more than 

 accelerate or retard this current, and it was only occasionally 

 that we were helped in our journey to the south and east by a 

 favouring stream. To the eastward of the Balleny Islands the 

 surface water is moving towards the north, but the absence of 

 vol. 11. x 



