James Croll — On the Glacial Epoch. 



307 



slope so small would be sufficient to discharge the ice, 

 — still we have twelve miles as the thickness of the 

 cap at the Pole. To those who have not been ac- 

 customed to reflect on the physical conditions of the 

 problem, this estimate may doubtless be regarded as 

 somewhat extravag-ant : but a slig-ht consideration will, 

 however, show that it would be even more extravagant 

 to assume that a slope of less than half a degree would 

 be sufficient to produce the necessary outflow of the 

 ice. In estimating the thickness of a sheet of con- 

 tinental ice of one or two thousand miles across, our 

 imagination is apt to deceive us. We can easily form 

 a pretty accurate sensuous representation of the thick- 

 ness of the sheet ; but we can form no adequate re- 

 presentation of its superficial area. We can represent 

 to the mind with tolerable accuracy a thickness of a., 

 few miles, but we cannot do this ia reference to the 

 area of a surface 2800 miles across. Consequently, in. 

 judging what proportion the thickness of the sheet 

 should bear to its superficial area, we are apt to fall 

 into the error of under-estimating the thickness. We 

 have a striking example of this in. regard to tho 

 ocean. The thing which im..presses us most forcibly 

 in regard to the ocean, is its profound depth. A mean 

 depth of, say, three miles produces a striking impres- 

 sion : but if we could represent to the mind the vast 

 area of the ocean as correctly as we can do its depth, 

 shallowness rather than depth would be the impression 

 produced. A sheet of water one hundred yards in. 

 diameter, and only one inch deep, would not be called 

 a deep but a very shallow pool or thin layer of water^ 

 But such a layer would be a correct representation of 

 the ocean in miniature. Were we in like manner to 

 represent to the eye in miniature the Antarctic ice- 

 cap, we would call it a thin crust of ice. Taking the 

 mean thickness of the ice at four miles,, the Antarctic 

 ice-sheet would be represented by a carpet covering 

 the floor of an ordinary-sized dining-room. Were 

 those who consider the above estimate of the thickness 

 of the Antarctic ice-cap as extravagantly great called 

 upon to sketch on paper a section of what they should 

 deem a cap of moderate thickness, ninety-nine out of 

 every hundred would draw one of much greater 

 thickness than twelve miles at the centre. 



The accompanying diagram represents a section 

 across the cap drawn to a natural scale ; the upper 

 surface of the sheet having a slope of half a degree. 

 No one on looking at the section would pronounce it 

 to be too thick at the centre, unless, he were previously 

 made aware that it represented a thickness of twelve 



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