122 E. H. L. Schwcirz — The Thickness of the Ice-Cap. 



floating glaciers gave only a depth of four times the height of the 

 glacier front, and a 300 feet iceberg would therefore be 1,500 feet 

 thick in all. Scott says the same thing for the Antarctic, where 120 

 to 150 feet icebergs do not touch bottom in more than 100 to 120 

 fathoms of water, giving a proportion of something like 1 : 5 for the 

 portions above and below water.' The discrepancy between the 

 estimated and observed proportions is explained by the experiment 

 performed by the " Challenger " in firing cannon-balls into an ice- 

 berg : while some layers reacted like solid rock, others were so soft 

 that the balls imbedded themselves, proving that a large proportion 

 of the berg was soft, spongy, and filled with air, thus making it 

 buoyant. 



Scott found that the Great Barrier ice rose at one point to 240 feet 

 above sea-level, but as some portions sank to nearly sea-level, these 

 greater elevations may safely be put down to pressure ridges ; the 

 average height was under 200 feet. The height of the ice-column 

 in the Great Barrier may give too low a figure for the mass of ice as 

 it leaves the land, whereas that of the Greenland glaciers, coming 

 steeply off" the land, probably gives too high a one. The point 

 I wisli to emphasize is that the whole resources of the Greenland 

 ice-cap and the great snow-fields of the Antarctic, can only produce 

 a sheet of ice in the valleys or places of greatest accumulation of 

 heights that are near the estimated possible maximum, namely, 

 1,600 feet. 



These estimations depend on the ice being 0° C. at the junction of 

 the sheet with the earth, and doubt has been thrown on this. Ice 

 only becomes colder than its melting-point by reason of radiation of 

 heat into space, and where this is stopped by great thickness of ice 

 — ice being a bad conductor of heat — the temperature of the ice will 

 rise from any degree below zero at the surface to 0° C, where the 

 heating eff"ect due to pressure comes into play. V. Drygalski has 

 given a large series of observations bearing on this question, and the 

 same rise of temperature is observed as in the case of the earth's 

 crust, only on a more exaggerated scale. The following are a few 

 extracts, temperatures in degrees Centigrade (Celsius in the 

 original) : — ^ 



Temperature ix the Ice ox the Great Kaeajak Glacier, 

 175 m. above sea-level. 



Gin. (-IS m.) 17-5 ft. (5-4 m.) 28-9 ft. (8-9 m.) 



SOth October, 1892 ... —13-7 ... -3-7 ... —2-9 



19th November, 1892 ... —13-7 ... —10-1 ... —7-2 



29th November, 1892 ... —19-2 ... —5-5 ... —5-2 



2nd February, 1893 ... —32-9 ... —6-1 ... —5-0 



14th April, 1893 ... —15-0 ... —9-0 ... —9-2 



The last figure in the third column is the greatest cold found 

 at this depth ; it was taken on April 14:th, and denotes the lag in the 

 temperature of the interior of the ice-sheet when the surface was 

 becoming warmer. These figures demonstrate sufficiently clearly 



1 "Voya,2:e of the 'Discovery,'" p. 410; London, 1905. 

 ^ "Greenland Expedition," pp. 451-2. 



