FORMED AT THE BOTTOM OF RUNNING WATER. 341 



modifies the radiation, 1 shall detail some observations I made on the 7th of January 

 last, incidentally in the first instance, but then extended, in reference to the obser- 

 vations on the ground-gru, which I was making at the time. Having occasion that 

 day to dig into recently hoed ground, in the middle of a garden, remote from shade, 

 the soil was observed to be frozen to the depth of four inches, by the clear frost, 

 which had continued from the 1st of January, with the trifling intermission above 

 mentioned. On digging into similar ground at the north base of a wall six feet 

 high, the soil was found, close at the foot of the wall, frozen to the depth of only 

 half an inch ; at a foot distance from it, about an inch ; at two feet, little more ; and 

 it was only at the distance of ten or twelve feet that it was frozen hard to the depth 

 of three inches. A similar modification of the effect of radiation was observed in 

 the shade of trees. Under the Scotch fir the soil, slightly covered with decaying 

 herbage, was not at all frozen ; although in similar ground similarly covered, but re 

 mote from shade, it was hard frozen to the depth of two or three inches. 



Now the ground-gru in the rivers was modified in a way strictly similar by the 

 effect of shade. The bridge of Alford, over the Don, is happily situated for illus- 

 trating this, being on one of the rapids, where the ground-gru is earliest and most 

 abundantly formed. While the other rapids, and the unshaded parts of this one, 

 were quite occupied by gru on both the 5th and 7th of January, spaces in the shade 

 of the masonry at this bridge were quite clear of it. It cannot be admitted as an 

 explanation of this fact, that heat may have been there laterally transmitted to the 

 water by contact with the piers and walls ; for if this took place, why then did the 

 clear spaces on the bottom narrow gradually towards the low extremities of the em- 

 banking walls ? Besides, the transmission of heat laterally had not hindered the for- 

 mation of surface-ice, in contact with a pier, on a piece of still water under one of 

 the arches. The modification of the radiation by shade was also exhibited in the 

 absence of all gru on the bottom, along the foot of the dense tuft of Phalaris grass 

 in the Leochal, where there could be no more transmission of heat laterally, than at 

 the general line of the grassy banks of this stream. 



The water, too, returning warmer from under the surface-ice, on the little pools at 

 the edge of one of the rapids, is another instance of the modification of the radiation 

 by shade. The thin white opake covering of hoar frost on the ice prevented radia- 

 tion, at least in a great measure, and the heat of the bed of the river, in the course 

 of continual transmission upwards, from strata not yet cooled to much depth by the 

 frost, finding no outlet by the radiation, was expended in heating the water by con- 

 tact. 



There was another phenomenon observed on the 5th of January, (although no 

 longer seen on the 7th, being then concealed by the immense formation of gru,) 

 which can be readily explained by the admission of the radiation of heat through 

 the water, and therefore goes to support the justness of the theory. The tufts of 

 water starwort, in the deepest and stillest parts of one of the pools, were the darkest- 



