148 EMISSION SPECTRA. 



what is known in regard to the more copious radiation of heat from dark surfaces. Anchor 

 ice never forms under a cloudy sky either by day or by night, no matter how severe the weather, 

 but it forms very rapidly under a clear sky at night. Anchor ice is readily melted off under a 

 bright sun. It seems highly probable, then, that radiation of heat supplies the necessary 

 cooling to the bottom of a river to form the first layers of ice, after which the growth or build- 

 ing up of the ice is aided by the entangling and freezing of frazil crystals, which are always 

 present in the water. 



The author found that during rapid ice formation the water becomes 

 slightly undercooled to the order of a few thousandths of a degree, and 

 that the ice which is found is in a very adhesive state. On the cessation 

 of cold weather the temperature of the water rises slightly above the 

 freezing-point and the ice gradually melts. Anchor ice rises from the 

 bottom in mild weather and also in extreme cold weather under the in- 

 fluence of a bright sun, when it is dangerous to small boats. It is also 

 known to lift and transport large boulders. On the other hand, a bright 

 sun prevents the water from becoming undercooled and the formation of 

 frazil ice. The author's conclusion that anchor ice is formed by radiation 

 rather than by conduction is practically the same as that of Farquharson 

 in 1841. 



At the request of the editor of the Monthly Weather Review, the writer 1 

 has inquired into the aforesaid conclusions, which at first seemed unten- 

 able. After considering various experimental data, it appears to the pres- 

 ent writer that the explanation of Farquharson and of Barnes accounts for 

 the observed phenomena better than any of the other theories propounded. 

 Thus the loosening of the anchor ice under a bright sun is simple enough 

 from the fact that water is transparent to heat-waves up to 1 /*. The 

 thickness of the layer of ice that must be melted in order to overcome the 

 adhesion to the rock surface must be of molecular dimensions. In addi- 

 tion to this, there is the tension on the rock surface due to the buoyancy 

 of the ice, which also tends to melt the ice. The explanation of the for- 

 mation of anchor ice is more difficult, and the author's statement that 

 "it is not to be supposed, because a substance like water has been found 

 to be highly opaque to the radiation from hot bodies, that it will be the 

 same for cold body radiation" is a little startling, and not very clear, for 

 the transmissivity of any region of the spectrum is independent of the 

 temperature of the source. However, if total radiation is meant, then 

 such an interpretation is possible. In "Investigations of Infra-red Spec- 

 tra" (Carnegie Publication No. 35), several examples, e.g., methyl iodide, 

 are given, illustrating the latter case. There is no evidence, however, for 

 saying that "it is probable that water possesses an absorption band for 

 shorter heat-waves, but may become perfectly transparent for the longer 

 heat-waves." It is known that water is exceedingly opaque to heat rays 

 from 4 to 8 ft followed by a more transparent region from 8 to 20 /* (this 



1 The Monthly Weather Review, p. 225, May, 1907. 



