i5o 



EMISSION SPECTRA. 



probably at absolute zero of temperature. The river-bed is radiating 

 energy into the water, and probably through it into space. Leaving out 

 of consideration the special nature of the emissivity of the two bodies 

 (water and river-bed), it has been established (see Drude's Optics, p. 462) 

 that the radiation from a "non-black body" is approximately proportional 

 to the square of the refractive index of the surrounding medium, which is 

 transparent, so that, from this standpoint, the emissivity of the river-bed 

 into the water would be greater than that of the water into the air. Of 

 course, if water were perfectly transparent, its emissivity would be nil, 

 and the problem would be less complex. Little is known concerning the 

 special nature of these two bodies, but from the fact that the anchor ice 

 separates so easily from the river-bed under a bright sun, it is evident that 

 the absorption coefficient of rock material is greater than that of water, 

 and hence, that its emissivity must also be greater. Hence, more energy 

 will be radiated from the river-bottom than from the water, into space, 

 the river-bottom will become the cooler, and finally a film of ice is formed 

 on it. During cloudy weather the temperature of the water-vapor in the 

 air is equal to or higher than that of the water and the river-bottom. There 

 is then an equality in the radiation, or an excess is being emitted from the 

 clouds to the earth. A certain amount will also be returned from the 

 clouds by reflection. Hence, the excess of radiation is toward the earth, 

 and since the temperature of the clouds is above the freezing-point no 

 anchor ice is formed. 



To sum up, from this elaboration of the author's explanation, just 

 quoted, of the formation of anchor ice, it will be seen that it is not only 

 possible, but also highly probable that the cause is to be attributed to the 

 greater emissivity of the substances forming the bed of the river, and to 

 the greater transparency of water to heat-waves than is generally supposed 

 to obtain for that substance. It is difficult to conceive that such a condi- 

 tion can exist, but the magnitude of the heat transfer, required to bring 

 about this ice formation, must be exceedingly small, and the explanation 

 given accounts for all of the facts observed. 



On a previous page the conclusion was reached that the energy re- 

 ceived from the moon will be the composite of the selectively reflected 

 energy of the sun and of the selectively emitted energy of the moon, which 

 is selectively transmitted by the earth's atmosphere. In the same manner 

 the earth would emit selectively in the region of 8 to 10 p and less than a 

 complete radiator at the same temperature. It is, therefore, hotter than a 

 complete radiator which emits the same amount of energy. All these ques- 

 tions are of extreme importance to the meteorologist who is concerned 

 with the cooling of the earth. It may be added, however, in conclusion 

 that the loss of radiation from the earth under the above conditions must 

 be exceedingly small. In the same manner the suppression and conser- 

 vation of energy in the region of 8 to 10 p. must be very small, so that it is a 



