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 speculæ, 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 phenomenon 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 wagon, 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 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 connexion. 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

1

velocity of the stream, little or none being found where there is no sensible current, seems in some degree to countenance the above idea.

I cannot learn that any experiments have ever been instituted on this subject, though it seems that they might easily be made by a person conveniently situated, and possessed of the requisite instruments. A careful examination, by properly. contrived thermometers, of the relative temperatures of the air, the water, and the bed of the river, and of the changes undergone by them during the above process, would probably go a great way towards solving the problem. I know no one better qualified for this undertaking than Mr. Knight, if he should at any future period have leisure and opportunity to direct towards it the same acuteness of observation and accuracy of investigation which have enabled him to make such important discoveries in the economy of the vegetable kingdom. And, if the explanation of this phenomenon should ever lead to results of any importance to the cause of science, I shall feel sufficiently satisfied, if it be deemed that I have been of any service in pointing out the way.

Blackburn, Lancashire, May 16th, 1818.

RICHARD GArnett.

ART. X. On the Sounds produced by Flame in Tubes, &c. by M. Faraday, Chymical Assistant in the Royal Insti

tution.

THERE is an experiment usually made in illustration of the properties of hydrogen gas, which was first described by Dr. Higgins, in the year 1777,* and in which tones are produced by burning a jet of hydrogen within a glass jar or tube. These tones vary with the diameter, the thickness, the length, and the substance of the tube or jar; and also with changes in the jet. They have frequently attracted attention, and some attempts to explain their origin have been made.

Nicholson's Journal, Vol. I. page 130.

After Dr. Higgins, Brugnatelli in Italy, and Mr. Pictet at Geneva, described the experiment, and the effects produced by varying the position and other circumstances of the jet and tube; and M. de la Rive read a paper at Geneva (published in the Journal de Physique, LV. 165.) in which he accounted for the phenomenon by the alternate expansion and contraction of aqueous vapour. That they are not owing to aqueous vapour will be evident from some experiments to be described. I have no doubt they are caused by vibrations, similar to those described by M. de la Rive; but the vibrations are produced in a different manner, and may result from the action of any flame.

I was induced to make a few experiments on this subject, in consequence of the request of Mr. J. Stodart, that it should be introduced at one of the evening meetings of the Members and Friends of the Royal Institution; and was soon satisfied that no correct explanation had been given. That the sounds were not owing to any action of aqueous vapour was shown by heating the whole tube above 212° and still more evidently by an experiment, in which I succeeded in producing them from a jet of carbonic oxide. That they do not originate in vibrations of the tube, caused by the current of air passing through it, was shown by using cracked glass tubes, tubes wrapped up in a cloth, and I have obtained very fine sounds by using a tube formed at the moment by rolling up half a sheet of cartridge paper, and keeping it in form by grasping it in the hand. The sounds have been accounted for, as well as their supposed peculiarity of production by hydrogen, by the supposition of a rapid current of air through the tube; but that this is not essential, is shown by using tubes closed at one end, and bell glasses, as described by Mr. Higgins, in his first experiment.

I was surprised to find, on my first trials with other gases, that I could produce those sounds from them which had been supposed to be generated exclusively by hydrogen and this, with the insufficiency of the explanations that had yet been given, induced me to search after the cause of an effect, which appeared to be produced generally by all flame.