A CONTRIBUTION TO THE STUDY OF ICE STRUCTURES 21 



the size of the individuals is often very large. During the winter the moister air from 

 below rises in the crevasse, and as it is chilled nearing the surface, crystals are deposited 

 on the walls. In summer-time, no doubt, additions are made from moist air entering 

 from above. So a spongy mass of crystals is developed which, in time, chokes the mouth 

 of the crevasse. 



Frost-ice, which forms at low temperatures, is of this class. The duration of any 

 particular frost-period is usually so short that in cases where the temperature is below 

 zero F. the deposition is scarcely perceptible to the eye, but is readily felt as a sort of 

 sand-paper coating on all exposed objects. Attention was particularly drawn to this 

 frost-ice on account of its interference with the lenses of the spectroscopic camera when 

 exposed for long intervals during the winter night. Plate VII, Fig. 1, is a photograph 

 of the front of the spectroscopic camera on an occasion when it showed an abundance of 

 frost-spicules. Frosts of the kind of which I am speaking took place at temperatures 

 below zero F. , that illustrated having formed at -30° F. 



Analogous with this class is the precipitation from a cloudless sky occasionally 

 experienced at sea -level during the winter, and even in the height of summer on the 

 plateau. Such falls are composed of minute, well-formed, clear crystals. The mean 

 dimensions of the particles of a fall during the winter at Cape Royds was found to be 

 one-hundredth of an inch. Many of the wonderful optical effects witnessed on occasions 

 in the Polar regions is due to the presence of these particles floating in the atmosphere. 



In conclusion it is to be remarked that the subject of vaporisation and redeposition 

 of ice is a most important one, for it is by such processes that snow becomes converted 

 into neve and is also largely accountable for the convertion of neve into ice. Loosely 

 packed snow holds a large volume of air which is ever taking up and redepositing ice 

 pari passu with fluctuations of the temperature and barometric pressure. Thus the 

 individual grains become increased in size and the pore space reduced in accordance with 

 the well-known laws governing recrystallisation under such conditions. 



