frost is so intense as materially to affect the water of a river, we may then expect to find ice at the bottom. Now this is certainly not the case, since the appearance we are treating of never occurs but under peculiar atmospherical circumstances; and rivers are frequently frozen over, and remain so for a length of time, without a particle of ice being visible at the bottom of their streams. I do not now profess to have developed this mystery, but merely intend to state the circumstances under which the phænomenon commonly takes place, as well as a few particulars connected with it, which perhaps are not generally known, and which may hereafter be serviceable as data for investigating the cause.

It is well known to meteorologists, that a severe frost in winter does not always commence in an uniform manner. Sometimes it begins with a gentle wind from the E. or N. E. and is at first comparatively mild in its operations, but afterwards gradually increases in intensity. Frosts of this kind are generally more lasting than others, and during such, I have not observed that any ice is generated at the bottoms of streams; though the deep and still parts of rivers are often frozen over by them to a considerable extent. At other times, during the continuance of the violent south-westerly gales, which are so prevalent in this country in the winter months, the wind frequently shifts on a sudden from S.W. to N.W. commonly about an hour before sun-set, and blows with great impetuosity in this direction, attended with a severe frost, and sometimes with a heavy fall of snow. The effects of this frost in places exposed to the wind are extremely rapid, so as to render the ground impenetrably hard, in about a couple of hours from its commencement. Situations that are not so much exposed seem comparatively little affected; at least I have repeatedly observed, that a small sheltered pond in a field was nearly free from icè, while the current of a large and rapid river at no great distance was nearly choked up by it. I believe that the phanomenon under consideration seldom occurs, except during such frosts as these; and the following are the principal circumstances connected with it, which I am able to state from my own observation.

It may be here premised, that ice of this description is seldom seen adhering to anything besides rock, stone, or gravel; and that it is more abundantly produced in proportion to the greater magnitude and number of the stones composing the bed of the river, combined (as will be further noticed) with the velocity of the current. I have been informed by a friend, that he has occasionally perceived it attached to solid wooden piles, at a considerable depth beneath the surface. of the water; but I never saw, or heard, of any on mud, earth or clay. It is not easy to ascertain the precise time at which the process begins to take place. It appears, however, almost invariably to commence during the first night of the frost, and probably within a few hours after sunset. On the ensuing morning, the first thing which strikes an observer is an immense quantity of detached plates of ice floating down the stream. Mr. Knight naturally enough supposed these to have been formed at the surface by the influence of the freezing atmosphere, and afterwards absorbed by the current; but I think that a minute inspection would have led him to form a contrary conclusion; viz. that they are first formed in the bed of the river, and afterwards rise to the surface. It is true, that none are to be seen in situations where there is no sensible current, and that they abound most in rough and rapid places. But on examining any stream of moderate velocity, yet smooth, equable, and free from all appearance of eddy or rippling, a great number of these plates of ice will be found adhering to the rock, stone, or gravel at the bottom. If they are watched with attention, they will visibly and rapidly increase in bulk; till at last, on account of their inferior specific gravity, aided perhaps by the action of the current, they detach themselves from the substances to which they at first adhered, and rise to the top of the water. The form of these pieces of ice is very irregular, depending, in a great measure, on the size and shape of the stones, or other substances, to which they were originally attached. Most of them seem to be of an oblong, or circular figure; they are generally convex on the upper surface, and have a number of thin lamina and spicula shooting from them in various directions, especially from their circumference.

Sometimes, when these floating pieces or plates meet with any obstruction in the channel of the river, they accumulate in such quantities as to cover the surface of the water, and become frozen together in one large sheet. But this kind of ice may be always readily distinguished from that produced in the usual way by the action of the cold air on the surface, which is smooth, transparent, and of an uniform texture. On the contrary, one of these conglomerated fields or sheets, is opaque, uneven, full of asperities, and the form of each separate plate composing it may be distinctly traced. In this situation they commonly assume the shape of irregular polygons, with angles somewhat rounded; a form apparently caused by the lateral pressure of the contiguous pieces.

On the river Wharfe, near Otley, in the West Riding of Yorkshire, is a weir, or mill-dam, where this phenomenon is sometimes manifested in a striking manner. This structure is of hewn stone; forming a plane inclined at an angle of from 35° to 50°, fronting the north, and extending from W. to E. to the length of 250 or 300 yards. When one of the abovementioned frosts occurs, the stone which composes the weir soon becomes encrusted with ice, which increases so rapidly in thickness, as in a short time to impede the course of the stream, that falls over it in a tolerably uniform sheet, and with considerable velocity: at the same time, the wind blowing strongly from the N. W. contributes to repel the water, and freeze such as adheres to the crust of ice when its surface comes nearly in contact with the air. The consequence is, that in a short time the current is entirely obstructed, and the superincumbent water forced to a higher level. But as the above-mentioned causes continue to act, the ice is also elevated by a perpetual aggregation of particles: till by a series of similar operations, an icy mound or barrier is formed, so high as to force the water over the opposite shore, and thus produce an apparent inundation. But in a short time the accumulated weight of a great many thousand cubic feet of water presses so strongly against the barrier, as to burst a passage through some weak part, through which the water escapes, and subsides to its

former level, leaving the singular appearance of a wall or rampart of ice three or four feet high, and about two feet in thickness, along the greatest part of the upper edge of the weir. The ice composing this barrier, where it adheres to the stone, is of a solid consistency, but the upper part consists of a multitude of thin laminæ or layers, resting upon each other in a confused manner, and at different degrees of inclination; their interstices being occupied by innumerable icy spiculæ, diverging and crossing each other in all directions. The whole mass much resembles in its texture the white and porous ice which may be seen at the edge of a pond or small rill, where the water has subsided during a frost.

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 S. W, 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 encrusted 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 phænomenon was in a stream of the river Air, 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 an approaching change of temperature; and the day following there was a partial thaw, attended with rain, the wind having veered from N. W. to S. W. 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 this period, though the thermometer frequently stood below 18° of Fahrenheit, and the estuary of the Tees, several miles below Stockton, where the spring tides rise from twelve to eighteen feet, was for more than 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 phænomenon does not merely depend on intensity of cold.

I confess myself unable to frame any hypothesis 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 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. We may easily conceive, that by a sudden change from a state of thaw to an intense frost, attended by a strong wind, the whole body of water in a river may become quickly cooled, and consequently diminish the temperature of the stone or gravel over which it flows: but to suppose that water, which is not itself at the freezing point, is capable of reducing the substances in contact with it, by means of a rapid and continual application of successive particles, so far beneath that temperature, as in process of time to convert the contiguous water to ice, seems not to accord very well with the usually received theory of the equilibrium of caloric. However, the fact that the quantity of ice thus produced is always greater in proportion to the superior