COOLING OP WATER. 163 



at a temperature of 40°, or about 8° abote the freezing point ; wlien thia 

 is the case, all circulation in tbe water ceases, because if the surface 

 ■water is then cooled still lower, it no longer continues to contract and 

 become denser, but on the contrary expands, so that it then remains 

 floating on the surface. It follows from this very interesting fact, that 

 when on a cold winter's night the surface of a pond is cooled, the whole 

 body of water sinks in temperature to 40°, after that, the surface only 

 continues to get colder, and a film of ice is soon formed, while the 

 water below continues at a temperature of 40°. In consequence of this 

 kind of circulation, and the facility with which it is produced, a body 

 of water is easily cooled down to within 8° of freezing, but when once 

 it has arrived at that point its further cooluig proceeds very slowly, 

 even though the cold becomes much more intense ; for the water below 

 is in fact protected from contact with the cold air by the film of ice at 

 the surface, and ice is so bad a conductor of heat that the freezing of 

 the water under the ice goes on very slowly ; in temperate climates ice 

 is seldom more than a few inches in thickness, and the water in deep 

 ponds not only never freezes, but, indeed, never falls in temperature 

 much below 40°. 



Water plants, therefore, axe, in. fact, preserved from cold by the 

 coating of ice which forms over the surface of the pond in which they 

 grow ; if the water is deep they are seldom injured ; but if the water is 

 shallow, and the cold long-continued, the whole depth of it will in time 

 freeze, and the plants will be more or less injured. Plants growing in 

 water thus walled over with ice are protected from all the three cooling 

 influences to which we have alluded ; but there are some circumstances 

 under which water plants suffer greatly, and from a very singular 

 cause, but one which, when looked into, is sufficiently simple and 

 intelligible. 



The surface of clear water does not become cold from radiation, but 

 from contact with cold and dry air ; consequently in a fine but very still 

 night it is much less rapidly cooled than the earth, which, in addition, 

 is exposed to the cooling influence of radiation. Under such circum- 

 stances it sometimes happens that the usual order of things is reversed,, 

 the bottom of the pond oooUng more rapidly than the surface ; on a 

 clear still cold night radiation sometimes occurs from the bottom of a 

 pond, the plants and soil ia which they are growing radiating towards 

 the sky just as if the water were not above them, and the consequence 

 is that they become very cold, in fact, some degrees below the freezing 

 point, though the water above them is still at 40°. This effect can only 

 happen in clear water, and on a night when there are no clouds, for the 

 same circumstances which prevent radiation from the surface of the 

 ground wiU also prevent its taking place from the bottom of a pond. 

 When plants under water are cooled by radiation, they soon become 

 encased in ice, and though the ice thus formed generally melts the next 



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