146 Analyses of Books. 



readiness with which crystals form on pointed and rough bodies in a 

 saturated saline solution. 3rd. The existence of a less impediment 

 to the formation of crystals, in the slower motion of the water at 

 the bottom, than in the more rapid one near, or at the surface. Mr. 

 Farquharson, however, considers that these propositions, neither 

 separate, nor combined, are adequate to account for the phenomenon, 

 and conceives that the question should be simply resolved into, 

 " Why is ice formed sometimes on the surface of winning mater, 

 and sometimes at the bottom ?" The ice found in this condition is 

 termed, by the Germans, Grundeis, and in Aberdeenshire, Ground- 

 gru ; gru being the term applied to snow saturated with, or swim- 

 ming in water. The author has seen it formed only when the tem- 

 perature of the whole mass of water was reduced to, or nearly, to 32°, 

 and when the temperature of the air was several degrees below that 

 point, and observed that it was preceded by a continuance, for some 

 time, of a clear, or very nearly clear, state of the sky. He brings 

 forward a number of observations made by himself, which are decid- 

 edly opposed to the explanations of Arago. At sunrise, on the river 

 Leochal and Don, when the temperature of the water was 32°, and 

 of the air 23°, much ground-gru was observed at the bottom of the 

 water; at 10 A. M., a cloud obscured the sky, and at 2 p. m. the air 

 was at 40° ; much gru rose to the surface, and floated down the 

 stream. Before sunset, the thermometer was 31°. On two sub- 

 sequent days, the temperature was lower, with a clear sky. The 

 bottom of the river was now much impeded by the gru ; and, what 

 is worthy of notice, the clear spaces of the bottom, at the piers, abut- 

 ments, &e. of the bridge, on the Don, still continued clear, but were 

 much encroached upon, on the sides next the streams, by the gru. 

 Next day a thaw occurred, when the thermometer rose to 47° ; the 

 rivers were cleared of ice and ground-gru, which floated away. On 

 the 2d day, the temperature was 29°, snow fell, and was entangled 

 in many parts of the rapids ; but there was no appearance of the 

 symmetrical cauliflower shaped ground-gru. On the same evening, 

 and two following days, the temperature fell to 23" and 21° ; the 

 sky was clouded; the rivers frozen over in many places, but no 

 ground-gru could be observed. The snow, however, which had 

 been entangled in the rapids disappeared to a great extent, obviously 

 floating away in the stream. He states also, that plants in the bed 

 of the stream were covered with gru, while none was observed in 

 the bed itself. The shaded parts of the stream were also free from 

 gru, while it was abundant in the free portions. The answer to his 

 original question, the author considers is, that ice is formed sometimes 

 on the surface of running water, and sometimes at the bottom, be- 

 cause frost sometimes takes place with a clouded sky, which is in- 

 compatible with radiation of heat from the bottom of the stream, 

 and sometimes with a clear sky, when that radiation takes place 

 through the water, in the same way as through air. The bottom is 

 thus cooled below the freezing point of water, before the water itself; 

 ice is formed on it, and its detacliment, by transmitted heat from 

 below, prevented, as long as the radiation continues. 



But, there is still a point of importance to settle. Why does de- 



