SCIENTIFIC KESULTS 111 



such cases the specific gravity of the ice in one part of the body will 

 differ from tliat in another. Such bergs will exhibit a marked sta- 

 bility, at least in their early stages, and the frequent number of flat 

 bergs observed floating with the original Greenland, sand-covered 

 tops, still uppermost, may be a corroboration of the above-described 

 conditions. 



The density of the ice in an iceberg may change after it has 

 detached from the ice sheet. This development is due to surface 

 weathering, a process which does not normally extend to a depth of 

 more than a few feet. Sverdrup has suggested to me that, accord- 

 ing to this view, an iceberg is probably composed of a core which is 

 densest, surrounded by layers in which the density decreases towards 

 the surface, depending upon the degree of porosity of the ice there. 



Wright and Priestley (1922) found the density of a sample of 

 ice taken from the Antarctic glacier to be 0.897. Barnes's (1928 

 p. 345) figure of approximately 10 per cent covolume of air taken 

 Irom an iceberg stranded off Twillingate, Newfoundland, should give 

 a density figure of about 0.825. Barnes, however, points out in a 

 letter to me that the density value of berg ice varies by a wide margin, 

 depending upon what part of the berg tlie sample wae. taken. 



The Coast Guard cutter Tampa in March. 1930. brought to Boston 

 a piece of ice taken from a berg found off Newfoundhmd. Determi- 

 nations of the specific gravity were made by means of the disj)]rtce- 

 ment method in a bath of kerosene at the Jefferson Physical Labora- 

 tory, Harvard University. From two trials with samples taken from 

 various parts of the piece the lowest density was 0.8977, the highest 

 0.9045. and the mean 0.8977. This figure agrees quite well with that 

 of Wright and Priestley for Antarctic glacial ice. but it is greater 

 than Ahhnanns's or Barnes's results. It is hoped that several more 

 den-ity determinations will be made from a number of samples to be 

 collected by the ice patrol from various parts of a berg as it disinte- 

 grates. Until more such data is assembled we can not state definitely 

 what is the figure representative of the mean density of an iceberg. 



A common question met in connection with the subject of icebergs 

 is: What proportions of a berg are above, and what proportions 

 are below, the surface of the sea? If we assume that the mean 

 specific gravity of iceberg ice is 0.8997, as compared with 0.9167 

 for pure fresh-water ice. we may arrive at the following estimate 

 of mass buoyancy. ^'^ The submerged portion of an iceberg floating 

 in sea w^ater of 1.02690 density, a figure representative of the average 

 density of the surface layers along the continental edge, is then about 

 0.8760 of the entire mass or, in other words, one-eighth of the mass 

 of a berg is above and seven-eighths below the surface of the sea. 

 These figures do not agree with Barnes (1927a. p. 98) who emphasizes 

 the amount of air imi)risoned in glacial ice. and states that because 

 of this a berg will float with about one-third of its mass above water. 

 It will require a greater number of density determinations than are 

 now available to ascertain this figure with accuracy. 



The fact that the icebergs typical of the Northern Hemisphere are 

 as a rule irregular in shape causes the question of mass flotation to 

 become a subject mostly of academic interest, and as a result atten- 



'•'^ \ cubic foot of iceberg ice weighs approximately 55 pounds. 



