It may be further observed that a frost of this kind is very limited in its duration, seldom lasting more than thirty-six or forty hours. On the morning of the second day after its commencement, a visible relaxation takes place in the temperature of the atmosphere. Usually before noon, the wind on a sudden shifts to the south-west, and a rapid thaw comes on, frequently attended with rain. What appears somewhat remarkable is, that during several hours after the commencement of the thaw, the production of ice at the bottom of rivers seems to go on without abatement, and upon examining a rapid stream, the stones over which it flows will be found at this period completely incrusted with the above description of icy plates. It seems evident from this that the bed of the river, which has been reduced below the freezing temperature, is not for some time affected by the change of the atmosphere. This may be in some measure illustrated by the well-known fact, that rain which falls upon a rock or stone wall, is frequently converted into ice, though the air and the ground are evidently in a state of thaw. Before the following morning, the ice of which we have been speaking generally disappears, being carried away by the current or dissolved by the thaw. The last time that I remarked this phenomenon, was in a stream of the river Aire, near Bradford, in Yorkshire, on the 1st of January, 1814. This instance did not precisely accord with what I have stated to be the usual circumstances of the case, as the frost then had existed several days without any previous appearance of this kind; but there were several indications of approaching change of temperature, and the day following there was a partial thaw attended with rain, the wind having veered from north-west to south-west. This thaw, however, did not continue long, and was succeeded by a frost which surpassed all within my recollection in severity and duration. Yet during the whole of the period, though the thermometer often stood below 18 degrees Fahrenheit, and the estuary of the Tees several miles below Stockton, where the spring-tides rise from twelve to eighteen feet, was for two months frozen over, so as to allow the passage of a loaded waggon, I could never perceive a particle of ice adhering to the rock or gravel, in the bed of the small and rapid river Leven in Cleveland, where I then resided. This circumstance seems decisively to prove that the phenomenon does not merely depend on an intensity of cold. I confess I am unable to frame any hypotheses respecting the above-mentioned facts which would not be liable to numerous and formidable objections. The immediate cause of the formation of the ice seems to be a rapid diminution of the temperature in the stone or gravel in the bed of the river, connected with the sudden changes in the state of the atmosphere, but it does not seem very easy to explain the precise nature of this connection.