304 In and Around the Morteratsck Glacier : 



ice ; but if blue ice is sufficiently illuminated it becomes white 

 ice ; therefore the experiment is not decisive. But the sun- 

 weathering of fresh ice demands time, whereas the human 

 eye, if prepared, can decide whether ice is blue or white in a 

 moment of time. When a fresh ice-surface is about to be 

 exposed it is of the greatest importance that the observer be 

 on the spot : but this usually depends on luck. 



Water a Protective Medium, Opaque to the Rays which 

 Disintegrate Glacier- Ice. 



There is, however, another way of arriving at the condition 

 of ice which is protected from the disintegrating effect of 

 radiation, namely, to enclose the mass of ice to be examined 

 as completely as possible in a medium which is opaque to, or 

 transmits only feebly the rays which produce the granular 

 disarticulation of ice. Such a medium is water, 



The polar seas, especially those of the southern hemisphere, 

 are crowded with icebergs. When they are of recent origin 

 they are of tabular form and cover a surface of many square 

 kilometres. 



In this condition they resemble flat boards which, when 

 they float on their sides, have great stability. But in the 

 course of time they begin to get dilapidated and to break up 

 into smaller units which may arrive at the state in which their 

 horizontal and vertical dimensions differ little from each other. 

 Such a berg is about as stable in one position as in another ; 

 in other words it is unstable. Any loss of substance can 

 bring about a disturbance of equilibrium which may cause the 

 berg to turn partially or completely over before it finds a new 

 position of equilibrium. The spectator who has the luck to 

 assist at an act of this kind, sees before him an intensely blue 

 berg floating in the sea surrounded by countless white bergs. 

 On the "Challenger" we had this good fortune. On one of 

 those wonderfully beautiful days of the Antarctic summer 

 the ship was engaged in the work of a " station," surrounded 

 by icebergs of the usual form and appearance. By the shifting 

 of the position of the ship or by an independent alteration of 



