1879.] 
Relations to that of the Glacial Epoch . 29 
ness of a few miles, but we cannot do this in 
reference to a superficial area 2800 miles across. 
Consequently, in judging what proportion the 
thickness of the sheet should bear to the 
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 
the ocean. That which impresses us most 
forcibly in regard to the ocean is its pro- 
found depth. A mean depth of (say) 3 miles 
produces a striking impression ; but if we could S2£ 
represent to the mind the vast area of the ocean »§ 
as corredtly as we can do its depth, shallowness ^ 
rather than depth would be the impression pro- £3 
duced. A sheet of water 100 yards in diameter, 2 § 
and only 1 inch deep, would not be called a deep |s. 
but a very shallow pool or thin layer of water. 
But such a layer would be a corredt representa- s-jj, 
tion of the ocean in miniature. Were we, in : P1 
like manner, to represent to the eye in miniature <§ “ 
the Antarctic ice-cap, even as 12 miles in thick- « g 
ness at the Pole, we should call it a thin crust of | 
ice. The mean thickness of the sheet would be * 
about 4 miles, and this would be represented by 
a carpet covering the floor of an ordinary- sized £ 
dining-room. Were those who consider the g* 
above estimate of the Antarctic ice-cap as extra- £ 
vagantly great called upon to sketch on paper a ^ 
sedtion of what they should deem a cap of mode- S 
rate thickness, ninety-nine out of a hundred would *» 
draw one of much greater thickness than 12 miles o 
at the centre. & 
The accompanying diagram represents a sec- ? 
tion across the cap drawn to a natural scale, the ^ 
upper surface of the sheet having a slope of half g 
a degree. No one looking at the sedtion would | 
pronounce it to be too thick at the centre unless ? 
he were previously made aware that it represented 
a thickness of 12 miles at that place. The sedtion, 
of course, is not intended to represent the adtual 
thickness of the sheet, but to show how liable we 
are to over-estimate a thickness proportionate to 
an area so immense. It may here be mentioned that 
had the sedtion been drawn upon a much larger scale 
— had it, for instance, been made 7 ft. long instead 
of 7 inches — it would have shown to the eye in a 
more striking manner the thinness of the cap. 
Section across Antarctic Ice-cap, drawn to a Natural Scale. 
