138 Royal Society. 



streams and rivers, differs in appearance from that which is formed nl 

 the surface; for, instead of assuming the shape of solid glass-like 

 plates,it has more theappearance of aggregated masses of snow, and is 

 com posed of small crystals of ice adhering together irregularly, either by 

 their sides or angles. Rivers are sometimes so choked up by accumu- 

 lations of ground-ice of this description, that they are not only impeded 

 in their course, but also raised considerably above their banks. While 

 in this state, a slight change in the weather will frequently occasion 

 the complete disengagement of this ice from the bottom ; so that, in 

 a ver'v short space of time, the river returns into its natural channel j 

 and then, although it maybe frozen at the surface, it continues to flow 

 over a perfectly clear bottom. All these phenomena are considered 

 by the author as perfectly explicable on the theory he advances, of 

 different degrees of radiation of heat occurring from the bottom ac- 

 cording to variations of circumstances. He conceives that when this 

 radiation takes place from the solid opake materials of the bed of the 

 stream, through the stratum of transparent water, congelation is in- 

 duced on that portion of fluid, already cooled down to the freezing- 

 point, which is in immediate contact with the radiating body. The 

 circumstances which, by favouring radiation, contribute to this effect, 

 are, principally, great clearness of the sky, and great transparency of 

 the water -, the bottom of the river being cooled below the freezing- 

 point sooner than the water which is flowing over it j and the ice, 

 formed at the bottom, remaining attached to it, as long as the heat 

 which is transmitted from below continues to be lost by radiation. 

 The formation of ground-ice is favoured by the intestine motions in- 

 cident to a rapid current; because the different strata of fluid, which in 

 still water would have arranged themselves, according to their specific 

 gravities, in the order most conducive to the congelation of the sur- 

 face, being continually mixed together, the whole body of water is 

 cooled more uniformly. 



The Society then adjourned over the Easter recess to meet again on 

 the 30th instant. 



April 30. — A paper was read, entitled, " Continuation of the paper 

 on the relations between the Nerves of Motion and of Sensation, 

 and the Brain ; and more particularly on the structure of the Medulla 

 Oblongata and of the Spinal Marrow." By Sir Charles Bell, F.R.S. 



The author enters into a minute anatomical investigation of the 

 structure of the spinal cord, and of its relations with the encephalon, 

 and with the origins of the nerves. He finds that the spinal cord is 

 constituted, in its whole length, by six pairs of columns, namely, two 

 posterior, two lateral, and two anterior ; each column being composed 

 of concentric layers, and invested with an external coating of cineri- 

 tious substance, and all the columns being divided from each other 

 by deep sulci, which penetrate nearly to the centre of the cord. On 

 tracing the posterior columns in their ascent towards the encephalon, 

 they are seen to diverge laterally at the calamus scriptorius, or bottom 

 of the fourth ventricle, and to proceed into the substance of the cere- 

 bellum. Each of these posterior columns is here found to consist of 

 two portions, the outermost being the largest - } and they now consti- 



