502 AGRICULTURAL REPORT.

state enlarge their volume at the moment of transition, and that the phenomenon exhibited by ice is only a conspicuous illustration of a general rule. Ice once formed is found to shrink more rapidly with aà diminution of temperature than any other substance on which ex- periments have yet been made.

The expansion of water and shrinking of ice serve to explain a variety of phenomena presented in the operations of Nature and the processes of the arts. Those who reside near the borders of rivers or fresh-water lakes are often startled during cold winter nights by explosions apparently as loud as those of discharges of heavy ord-

nance. These are produced by the rupture of long lines of ice—the

gradual shrinking which has been going on during the reduction of temperature tending to bring the whole mass into a state of tension, which is relieved by the sudden giving way along the line of least strength. Tam informed by Captain Meigs, who has paid particular attention to the cracking of ice on Lake Champlain, that it most frequently takes place in the narrower parts of the lake—the shrink- Ding of portions on each side of this line of least resistance tends to separate the two masses. The water sometimes rises in the cracks thus formed, a new freezing takes place, and when tlie weather mode- rates and the field expands to its original dimensions, it becomes too large for the area it covers, and long lines of ridges are thrown up. Ksimilar effect is sometimes produced on the surface of damp ground subsequently frozen. During the winter of 1856 and 1857, we received accounts of injury done to several brick houses by the separation due to the shrinking of the surface, passing through the foundation of the edifice, and extending up along the walls. We might infer, from the principles already stated, that the line of sepa- ration would, in preference, pass through a house, as this is the direc- tion of least resistance, for the cellar may be considered as a line of esure between the two masses of earth, or a crack already com- menced..

During a very cold night, when the temperature is rapidly dimin- ishing, and the ground covered with snow, slightly encrusted on the surface by previous thawing and freezing, a continued series of minute explosions may be heard, depending in frequency and loudness upon the thickness or thinness of the crust. In some cases, it resembles a crackling, and at others a series of distant, though not loud or sharp explosions. 1

There is a phenomenon connected with ice, in rivers, which has given rise to much discussion as to its cause. I allude to the freezing which takes place at the bottom of running streams, where, in some cases, it remains until it is separated by its buoyancy and rises to the surface. It presents a peculiar angular appearance, and is sometimes known by the namé of anchor ice. Its formation appears to be an exception to the general rule of the freezing of water, which, on account of the decreasing density, usually takes place at the surface. It was at first supposed that it was due to the radiation of heat through the clear water above; but Arago has shown that this expla- nation cannot be the true one, since rays of low temperature cannot

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