[BARNES] ANCHOR-ICE FORMATION 91 



v'hole mass of water was reduced to, or nearly to 32° Fahr., and when 

 the temperature of the air was several degrees below that point. I 

 liaA'e observed it in invariable condition, that it was preceded by a 

 continuance, for some time, of a clear, or very nearly clear, state of 

 the sky. 



This is at variance with another observation of Desmarest, quoted 

 by M. Arago, that " when, in consequence of a cloudy sky, the atmos- 

 pherical temperature experiences little variation throughout the day 

 and night, the ice at the bottom of the water uniformly increases 

 every twenty-four hours; on the contrary, when the sun shows itself, 

 the ice does not increase during the day." It is the fact, that while 

 it is forming under the continuance of a cloudless sky, its increase 

 is impeded during the day. It may be possible, amidst the infinite 

 variety of measures of cold that may exist at the time, that the increase 

 of the gru may go on for a little time after the sun has been obscured 

 by a thin cloud; but I have always seen, that when a densely clouded 

 state of the sky supervened, and continued for the space of even only 

 twenty-four hours, the gru became detached from the bottom, and floated 

 down the stream. Should the temperature of the air continue low, 

 with the clouded sky, or get lower, the ground-gru is not renewed, 

 but the river is speedily frozen over at the surface. It is, in fact, a 

 matter of frequent occurrence, in frosty winters, that our rivers, filled, 

 and so impeded, by ground-gru, as to be raised above their banks; 

 are found returned into their natural channels, and there frozen over 

 at the surface, but flowing over a clear bottom, in a space of time so 

 short as to appear very wonderful to those who have not investigated 

 the cause. The process is named, by the country people, the flitting 

 of the ice. In opposition to the observation of Desmarest, and in 

 confirmation of those which I have made, on this point, I may refer 

 to the Eev. Mr. Eisdale, who, not satisfied with the explanations of 

 ]\I. Arago, has published one of his own, in the Edinburgh New Phil' 

 osophical Journal, vol. xvii, p. 167. His explanation appears equally 

 unsatisfactory, as will be shown afterwards ; but the part of his state- 

 ment we have to do with here is his notice of this observation of 

 Desmarest. The formation of the ground-gru, under a cloudy sky, 

 is so much at variance with the information which Mr. Eisdale had 

 received, that he resolves Desmarest's '' cloudy skv " into " an atmos- 

 phere loaded with hoar frost, and rendered hazy by its condensation." 

 The state of the air, in respect of being windy or calm, deserves also 

 to be noticed. The ground-gru occurs most frequently during calm, 

 with a deposition of hoar frost upon the ground at the time; and 

 * Page 172. 



