PHYSICS 



was observed at Clear Lake. On November 3, 1908, a 

 large heap of chips of ice were thrown out from a shaft. 

 The chips were of hard clear ice, and of all sizes up to 

 several inches in diameter. On February 24, 1909, it 

 was found that prismatic structure had developed in the 

 heap, making it quite like the general surface of the lake, 

 which we supposed to have been derived from snow. The 

 prisms were most distinct at the surface, while within the 

 mass traces of the original chips were still visible. 



The mosaic design showing on the surface of prismatic 

 ice was not uniform, nor yet was it irregular. It built up 

 patterns, the prisms arranging themselves round centres, 

 as well shown in Professor David's photographs. As the 

 lake surfaces were more and more smoothed by ablation 

 the mosaic changed in character. The prismatic structure 

 became less distinct, and at some depth gave place to rows 

 of bubbles arranged in curvilinear designs, the relation of 

 which to the original hexagonal mosaic was not apparent. 



About midwinter a curious optical effect was noticed 

 at Blue Lake. The surface of the lake was almost entirely 

 prismatic, but had been much smoothed by ablation, and 

 was marked by ripples like those of the sand of the sea- 

 shore, but much larger. On walking across the lake the 

 moonlight was constantly glancing apparently from very 

 large plane crystalline surfaces, several inches in diameter, 

 inclined at a low angle to the horizontal. On touching 

 these spots it was evident that there were no large crystals 

 on the surface, but only the smooth-rounded ripples. The 

 internal structure of vertical prisms made it unlikely that 

 these broad crystalline surfaces would exist within the ice. 



In the heat of summer the prismatic layer on the sur- 

 face of Blue Lake became greatly eroded, large holes as 

 much as two feet in depth being formed. Parts of the 

 lake were unsafe to walk on, the loosened prisms crumbling 

 away under foot. 



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