FORMED AT THE BOTTOM OF RUNNING WATER. 333 



Alford, built of granite over the Don, in the middle of one of the rapids. At this 

 rapid, the whole bottom, with the exceptions to be immediately stated, was covered 

 with silvery gvu, appearing from two or three to five or six inches deep. My atten- 

 tion was particularly directed to the exceptions, as throwing a clear light on the 

 question of the radiation of heat from the bottom. Round each of the piers, and in 

 front of the abutments of the bridge, there was a space quite clear of all frozen mat- 

 ter, excepting at a side of one pier under an arch, where a piece of very still water, 

 caused by an obstruction at the bottom, was covered by clear sheet ice. On the 

 south side of the river, two embanking walls, one up and the other down the stream, 

 each twelve yards long, are built in a line with the water-courses of the abutment. 

 Close to the bridge these walls are eight feet high from the bottom of the stream, 

 but as they recede from the bridge the masonry slopes gradually to a lower level, till 

 the extremities are little above the level of the water. The bottom in front of these 

 walls was clear of ground-gru, as well as that in front of the abutments ; but the 

 breadth of the clear space in front of the walls narrowed gradually towards their ex- 

 tremities, in proportion as the masonry became lower, till at the extremity of the 

 downward wall especially, which ends at a sloping gravelly bank, the gru came to 

 the edge of the water. The space of the bottom clear of gru was about five or six 

 feet broad at the high parts of the walls next the bridge ; and the water runs on the 

 place at the medium depth and velocity of the rapid. There was another clear space 

 in the bottom of this rapid. About twenty-five yards above the bridge there is, in the 

 middle of the stream, a piece of still water, caused by an elevated bed of gravel, just 

 below it, over which the stream is very shallow. The still water, for an extent of two 

 or three square poles, was covered with sheet ice, and that again covered by a very 

 thin, but white, opake deposition of hoar frost. From under this ice the water, flow- 

 ing rapidly over the gravel bed below, had no ground-gru for a space of eight or ten 

 yards downwards. 



Above this rapid, a pool of moderate stillness, about three or four feet deep, ex- 

 tends a hundred and fifty yards in length. Over the bottom of this there were 

 scattered, in an irregular manner, many cauliflower-shaped clusters of silveiy gru, 

 most of them very small, and none that were observed covering more of the bottom 

 than a square foot or two at one place. In the deepest and stillest part of the pool 

 there were several tufts of water starwort, with sooty-coloured decaying leaves, form- 

 ing the darkest-coloured objects seen at the bottom. These were all densely tangled 

 with fringes of silvery gru. At the head of the pool, where the velocity acquired by 

 the water in the rapid immediately above it was not yet greatly diminished, an ap- 

 pearance of a different kind presented itself. There are here several large stones in 

 the bed of the stream, but none of them projecting above the water. On the faces of 

 these opposed to the stream there were seen quantities of gru of a different aspect 

 from that further down. It was not arranged in the same cauliflower shapes, but in 

 angular masses, like wreaths of snow blown by the wind. It wanted, too, the silveiy 

 glance of the other, and had more the appearance of a pale ash-coloured mud. On 



