FORMED AT THE BOTTOM OF RUNNING WATER. 331 



It will be better here also to state, generally, the conditions of temperature and 

 phases of the weather under which the ground-gru is formed. I have seen it occur 

 only when the temperature of the whole 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 have observed it an 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 conse- 

 quence of a cloudy sky, the atmospherical 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 con- 

 tinuance of a cloudless sky, its increase is impeded during the day. It may be pos- 

 sible, 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 tem- 

 perature 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 im- 

 peded, 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 Jlittlng 

 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 Rev. Mr. Eisdale, who, 

 not satisfied with the explanations of M. Arago, has published one of his own, in the 

 Edinburgh New Philosophical Journal, vol. xvii. p. 167. His explanation appears 

 equally unsatisfactory, as will be shown afterwards ; but the part of his statement 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 sky" into 

 " an atmosphere 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 this was the condition of matters during the ob- 

 servations now to be detailed. But it also occurs during a frosty wind, when there 

 is no hoar frost, which is formed only in a calm state of the atmosphere. The forma- 

 tion of the gru during wind, and consequently without any deposition of hoar frost 

 on the ground, is especially to be noticed in reference to Mr. Eisdale's explanation, 



* p. 172. 



