On the Structure of Ice. 337 



the lake, and other modifying circumstances ; and I desine 

 liere to advert especially to the fact, that although it is cou- 

 strueted of successive horizontal additions beneath, when it 

 decays in spring it consists of vertical prisms, penetrating 

 its whole thickness, and standing side by side, like the 

 columns of a basaltic cliff ; which, in their mode of forma- 

 tion, have, I imagine, a close analogy. Dr Slagintweit, In- 

 formed me, that neither the ice nor the basalt forms exact 

 prisms, the angles never having the precise measurements of 

 true crystals. In this condition, the ice may be strong 

 enough to support a considerable weight ; and I have travel- 

 led over it with a large party on several occasions, when 

 the prisms on whicli the foot rested were depressed at every 

 step, and a pointed stick could be driven through the whole 

 thickness into the water beneath, with as much ease as 

 into a bank of snow. The ice then, in fact, presents the 

 physical characters of a semifluid mass, as pointed out by 

 Professor Forbes, its parts being moveable on each other, 

 not only vertically, but, as in the case of travelling glaciers, 

 capable of gliding past one another horizontally. 



In spring, when the action of the sun-light is very power- 

 ful, an incipient thaw takes place at mid-day on the surface 

 of the snow, which, on freezing again, acquires a glassy 

 crust. As the season advances, but while the temperature 

 of the air is still even at noon far below the freezing point, 

 the crust in clear weather becomes penetrated in the direc- 

 tion in which it is struck by the sun's rays at mid-day by 

 innumerable canals, and finally crumbles into a granular 

 mass like the firn of the high Switz glaciers, that crackles 

 under the f^et as soon as the son sinks towards the horizon. 

 This firn is not universal ; it is more common within the 

 Arctic circle, and in situations where there seems to have 

 been originally a certain looseness in the texture of the snow, 

 and where its surface is so much inclined that the sun's rays 

 do not fall on it obliquely about noon. 1 did not notice it in 

 any quantity on the level surface of a lake. 



2. Rapid Evaporation of Snow and Ice. 

 J^ The rapid evaporation of snow and ice in tjie, winter and 



VOL. Jill. NO. CIV. — APRIL 1852. Y 



