ON WATER. 225 



haps, is that of ice on water. In all the temperate and 

 colder latitudes this is a common winter phenomenon 

 occurring whenever the thermometer falls below 32, and 

 most rapidly in clear, dry, and serene conditions of the 

 atmosphere. Every one who has watched by the stagnant 

 pool must have observed the first formation of ice a creep- 

 ing or shrinking, as it were, of the surface occasioned by the 

 incipient ice-crystals, which shoot hither and thither, inter- 

 lace, and coalesce till a thin continuous crust has gathered 

 over the whole. This is the first film ; and as freezing 

 takes place from above,* every successive film is formed 

 more slowly ; the ice-crust as it thickens protecting more 

 and more from the cold the water that lies below. Indeed 

 it is often curious to observe how little will obstruct the 

 radiation of heat and prevent the formation of ice. An over- 

 hanging tree, a few leaves drifted over the first film, or even 

 a cloudy sky, is sufficient to retard or obstruct ; and though 

 a clear and serene sky be in general most favourable, yet an 

 air of dry wind to remove any superincumbent vapour will 

 materially promote the operation. As the frost continues 

 the ice thickens, but not indefinitely, for in water of suffi- 

 cient depth this thickening acts as a barrier to its further 

 increase, and even in the coldest regions it is only the shal- 

 lower waters that are ever completely converted into ice. 

 Besides the ice that forms on the surface of fresh water, 



* This principle of freezing from above is not sufficiently taken advan- 

 tage of by our skaters, curlers, and ice-storers. Instead of waiting till 

 the frost has produced a sufficient thickness of ice in the natural way 

 a thing never to be depended on in our uncertain climate they ought 

 to have the water let by degrees over the surfaces of the already formed 

 ice, and in this way its thickness and strength would be rapidly aug- 

 mented. We believe the celebrated ice of Lake Wenham, now so largely 

 used in and exported from North America, is treated in this way each 

 successive surface being scraped and cleaned before the next overflow 

 of water. For hardness, transparency, and general beauty of mass, the 

 Wenham ice is unrivalled. 



