or circular figure; they are generally convex on the upper surface, and have a number of laminae and spiculae shooting from them in various directions, especially from their circumference.  Sometimes when those 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 generally 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 milldam 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 degrees to 50 degrees, fronting the north and extending from west to east, to the length of 250 or 300 yards.  When one of the above-mentioned frosts occurs, the stone which composes the weir soon becomes incrusted with ice, which increases so rapidly in thickness as in a short time to impede the course of the stream, which falls over it in a tolerably uniform sheet, and with considerable velocity; at the same time, the wind blowing strongly from the north-west, contributes to repel the water and freeze such as adheres to the crest of the weir 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 bank, 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 solid consistency, but the upper part consists of a multitude of thin laminae or layers resting upon each other in a confused manner, and at different degrees of inclination, their interstices being occupied by innumerable icy spiculae, diverging and crossing each other in all directions.  The whole mass much resembles 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.