APPENDIX I. 



THE EFFECT OF THE SURROUNDING MEDIUM UPON THE 

 EMISSIVITY OF A SUBSTANCE. 



In most of the problems in radiation from substances the surrounding 

 medium is air, and no account is taken of the refractive index, which is 

 taken as unity. In a remarkable research on "The Formation of River 

 Ice with Special Reference to Anchor Ice and Frazil," by H. C. Barnes, 

 it is shown that the surrounding medium plays an important part in the 

 rate of cooling of the earth's crust. 



In Canada, as well as other localities having high latitudes, three kinds 

 of ice are observed, viz, sheet or surface ice, frazil ice, and anchor ice. 



Surface ice is found only in still water, and is caused by the loss of 

 heat to the cooler atmosphere, by radiation and conduction from its sur- 

 face. Thickening of the ice-sheet takes place downwards by conduction 

 of heat through the ice to the air. 



Frazil ice is the French-Canadian term for fine spicular ice, from the 

 French for forge-cinders, which it is supposed to resemble. It is found in 

 all rivers or streams flowing too swiftly for the formation of surface ice. 

 A dull, stormy day, with the wind blowing against the current, is produc- 

 tive of the greatest amount of frazil ice, which, like anchor ice, has a 

 tendency to sink upon the slightest provocation, and to follow submerged 

 channels until it reaches a quiet bay. Here it rises to the under side of 

 the surface ice, to which it freezes, forming a spongy growth, attaining 

 great thickness; in some cases the author observed a depth of 80 feet of 

 frazil ice. 



Anchor ice, as the name implies, is found attached or anchored to the 

 bottom of a river or stream, and often attains a thickness of 5 to 6 feet. 

 It is also called ground-ice, bottom-ice, and ground-gru. In a shallow, 

 smooth-flowing river we are more likely to have anchor ice formed in 

 excess, whereas in a deep and turbulent stream we are likely to have more 

 frazil. In a river 30 to 40 feet deep anchor ice is almost unknown, al- 

 though large quantities of frazil are met with. 



Barnes remarks that 



The various facts of common observation in connection with anchor ice points to radia- 

 tion as the primal cause. Thus it is found that a bridge or cover prevents the formation of 

 anchor ice underneath. Such a cover would act as a check to radiation, and reflect the heat- 

 waves back again to the bottom. Anchor ice rarely forms under a layer of surface ice covered 

 with snow. It forms on dark rocks more readily than on light ones, which is in accord with 



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