NOTES ON ANCHOR ICE. 179 



—the granular ones by agitation and consequent eeration. If we 

 refer to the effect of friction upon the sides and bottom of the chan- 

 nel — giving the least velocity to the current at those points, we have 

 the retarding cause where the ice formation takes place, and yet a 

 disturbing cause which independent of its submerged position may 

 be sufficient to produce the granular formation. If the cold super- 

 stratum of the flowing water (which from its specific gravity at the 

 temperature under which anchor ice is formed, confines the substratum 

 in situ) may be supposed to act upon the bottom in the same manner 

 as the colder atmosphere upon the surface water, — the principal point 

 of difference would be whether the air disengaged in the process of 

 freezing would pass ofF, or enter into any new combination and form 

 anchor ice as snow is formed in the atmosphere. There is much 

 similarity between saturated snow and anchor ice. Heavy snow- 

 storms, when the water is very cold, produce the same effect upon the 

 river as anchor ice : the snow does not melt but, descending the 

 current, passes under the solid ice and clogs up the channel. The 

 specific gravity of saturated snow and anchor ice appear to be equal 

 and almost identical with that of the water. 



One consequence of this peculiar form of congelation may be briefly 

 referred to. The great rivers of Canada, the St. Lawrence and the 

 Ottawa, with the large majority of their tributaries, are terrace-like in 

 their profile, as contrasted with the easy and almost uniform slopes of 

 the Mississippi and its branches. ' At the outlets of all our lakes, 

 large and small, there are rapids with open water in winter to a greater 

 or less extent. During the most intense cold — 20*^ to 30" below zero 

 — this open surface is covered with white fog or mist, like frost rime, 

 completely hiding the dark water which is beyond the snow-covered 

 bordage and appears to extend across the river — a deception which 

 has lured many an unwary traveller to a frightful death. 



While the surface of these lakes and rivers is covered with ice, and 

 the earth with snow, with the sun almost powerless, the amount of 

 latent heat disengaged in the formation of anchor ice and sent up 

 from these numberless breathing holes may give a powerful check to 

 the duration of that severe temperature under which this peculiar 

 description of ice is so abundantly formed. 



The unexpected rise of the St. Lawrence above the Lachine Rapids 

 in January, 1857, suggests some questions of moment. Suppose the 

 «old term had continued another day, would the growth of anchor ice 



