Rev. Mr Eisdale's Observations on Ground-Ice. ITl 



form proper points of attachment to facilitate the formation of 

 icy crystals. This is, in fact, the same answer that was given 

 to my theory, in some of the newspapers, two years ago ; but it 

 is altogether inadequate, for this plain reason^ that, according to 

 it, the phenomena of ground-ice ought to appear in every hard 

 frost, when the water reaches the requisite temperature. But 

 so far is this from being the case, that in the hardest frosts 

 which we have ever seen, not a particle of ground-ice was found 

 in the river. Take, for instance, the very severe frost of 1813- 

 14, when the Tay was frozen over for many weeks, yet no 

 ground-ice was to be seen. Some gentlemen present may re- 

 member to have skated down the stream and through below the 

 arches of the bridge, whilst the ice every where was clear as 

 crystal, and the bed of the stream entirely free from the white 

 spongy ground-ice. 



Some now present will probably recollect that tjie theory 

 which I proposed, as a solution of these phenomena, was founded 

 on information which I had received from country people, and 

 others, whose operations depended on water-wheels, and whose 

 interests forced them to attend to appearances, which might 

 pass unheeded by others. The sura of their information was, 

 that the ground-ice was never formed but after a heavy rioiie, 

 or hoar-frost. If this is the fact, the explanation is obvious. 

 The hoar-frost, which is congealed moisture, precipitated from 

 the atmosphere, and falling into the river when the water is 

 cooled down to the freezing-point, cannot be dissolved. It re- 

 tains in the water the very shape in which it descends from the 

 air. When these small crystals fall on a deep unfrozen pool, 

 the water being above the freezing-point, the particles melt and 

 are incorporated with the water ; but in a shallow and agitated 

 stream, almost the whole water is brought, in succession, into 

 contact with the intense frost, and may thus be cooled down to 

 the freezing-point to the very bottom of the stream, before even 

 a pellicle of ice is formed on the stagnant pool. All the par- 

 ticles of hoar-frost, then, or frozen vapour which fall on such 

 a stream will remain unmelted ; and being tossed in all direc- 

 tions by the agitations of the current, will be brought into con- 

 tact with the rocks, or other substances projecting from the 

 bottom, to which they will readily adhere, and form a nucleus 



