CONDITIONS OF CONTINENTAL ICE. 209 



From the fact that the ice forming the upper layers 

 of the icebergs is less dense than that of ordinary ice, 

 Sir Wyville Thomson estimates that as much as one- 

 seventh part of the berg may be above water-line. 

 But, for the following reasons, I am unable to agree 

 with this estimate. It is true, as he remarks, that the 

 white ice which forms the upper portion of the berg is 

 less dense than ordinary ice, being composed of recent 

 snows; but, on the other hand, this will be counter- 

 balanced by the greater density of the lower portions 

 of the berg, which have been subjected for ages to 

 enormous pressure. I hardly think that there is any 

 good reason to conclude that the mean density of the 

 bergs is much under that of ordinary ice, namely, 

 092.* 



But even if we admit that as much as one-seventh 

 of the berg is above water, still a berg 500 feet in 

 height would be 3500 feet in thickness, and one 600 

 feet would be 4200 feet thick, while one 720 feet high, 

 of the tabular form, would be 5040 feet, or nearly a 

 mile in thickness. 



It would not, of course, be safe to conclude that the 

 thickness of the ice below water bears always the 

 same proportion to the height above. If the berg, for 

 example, be much broader at its base than at its top, 

 the thickness of the ice below water would bear a less 

 proportion than that indicated by the difference of 



* It is true that, from observations made ("Quart. Journ. Geol. 

 Soc, Feb., 1877) on the density of ice in Disco Bay, Mr. Amund 

 Helland found that, in consequence of the amount of air-bubbles 

 contained in the ice, its density was only 0*886, and from this he 

 concluded that one-seventh of the bergs was above water. But he 

 does not state at what part of the berg his specimens were taken. 

 If they were taken from near the top, or even at the water-line, it 

 might have been expected that the density would be very consider- 

 ably under that of ordinary ice. 



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