6 
Thickness of the Antarctic Ice, and its [January, 
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, 0*92.* 
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 specific gravity of ice and 
water. But a berg such as that recorded by Captain Clark, 
500 feet high and 3 miles long, may be relied upon as 
having the proportionate thickness under water. The 
same may be said of the one seen by Captain Smithers, 
which was 580 feet high, and so large that it was taken for 
an island. 
It maybe here remarked that a berg does not stand higher 
out of the water because the lightest side happens to be 
uppermost. The height above water is determined by the 
mean density of the berg, and is the same no matter how 
the various densities may be distributed through the mass. 
It would be the same though the berg were turned upside 
down. This follows as a necessary consequence from the 
fadb that the amount of water displaced by the berg is 
equal to its weight, and of course it is the same whatever 
side be uppermost. 
To evade the force of the evidence derived from the testi- 
mony of the icebergs, it is asserted by some that the heights 
thus recorded are mere guesses, and not the result of abtual 
measurement. But such an opinion is in direct contradic- 
tion to the express declaration of Admiral FitzRoy, who 
* It is true that, from observations made (Quart. Journ. Geol. Soc., Feb., 
1 877) 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 o'8S6, 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 expe&ed that the density would be very considerably under that 
of ordinary ice. 
