Annwersary Address, 235 



Christinas week, the thermometer was about S*" or 3° below 

 zero, at places in the valley of the Tweed, it sank to 7° or 

 8° degrees below zero. It was commonly supposed that the 

 dampness of places near water, was the cause of greater 

 damage there to vegetation ; — ^but from the above observations 

 and others elsewhere, it appears, that places at low levels are 

 always marked by greater cold in winter. Whether damp- 

 ness also contributes to the effect, has not been ascertained by 

 any accurate observations ; — and some doubt is thrown upon 

 the supposition, by the fact, that at Dunse Castle, the Portugal 

 laurels close to the lake there, were not affected. That low 

 places in winter should be colder than those in the same dis- 

 trict, at a higher level, within certain limits, follows from the 

 principle, that as air becomes colder it becomes heavier, and 

 therefo^-e flows to low situations. 



I took notice of a circumstance, during the cold of last 

 Christmas, which I may be allowed to mention here, for the 

 sake of recording it, that the Tweed at Milne Graden was 

 frozen over, with ice from 9 to 10 inches thick, and that 

 fully three weeks elapsed before it broke up. In the body of 

 the ice, I observed a number of extraneous articles, such as 

 weeds which had grown at the bottom of the river, pebbles 

 about the size of walnuts, and a fresh water mussel, with both 

 shells entire, and the animal alive. At this place the river is 

 about 12 feet deep, and at first I felt some difficulty in 

 accounting for the phenomenon. It must have been caused, 

 as it appears to me, by the ice being formed at the bottom of 

 the river. The upper surface of the water would of course first 

 be cooled down to the freezing point, and by intermixture, the 

 whole body of water below, would gradually be cooled also. 

 Now ice would first form, in such circumstances, at the bot- 

 tom, among stones, weeds, and other objects, which besides 

 obstructing the movement of the water, would serve as nuclei 

 for crystallization. But ice being lighter than water, — the 

 ice when it had accumulated in a sufficient quantity, would 

 become buoyant, and rise, carrying up with it small pebbles 

 and other bodies at the bottom to which it had attached itself. 



Qq 



