333 POPULAR LECTURES AND ADDRESSES. 



another tree which was found fixed in an iceberg 

 that floated across the Noith Pole, and there is the 

 mud which was found on it. The wood is evidently 

 a piece of a fir tree, with branches chiefly on the 

 one side. 



I want now to consider the physical properties 

 of ice, so as to learn what limits, if any, these may 

 give to a possible thickness of floating ice or of an 

 icecap. Can ice stand in the form of any ice- 

 berg 1,500 feet thick? Icebergs have been said 

 to stand at a height of 600 or 800 feet : but 

 evidence is wanting to justify such estimates. 

 The highest iceberg recorded is 700 feet, but 

 there is a doubt whether it was even so high as 

 that. What then would be the physical condition 

 of a mass of ice at the North Pole ? Think of the 

 ice with snow falling on it at the rate of 3 feet 

 annually. (I always reckon in feet of water, so 

 that if I speak of 3 feet of snowfall, I mean sufficient 

 to form 3 feet of water when melted.) 



In considering this question I must first call your 

 attention to Forbcs's celebrated theory of the vis- 

 cosity of ice, and his viscous theory of glaciers. 



