METEOROLOGXY. 503

pass through water, and hence no such radiation can take place. A more probable explanation has been given, I think, by the same author, in referring it to the fact that still water can be reduced be- low the freezing point without congealing, and that it will immediately be converted into ice if a bit of solid matter be thrown into the vessel in which the experiment is made, which may serve as a nucleus for the crystallization. When water in this state is passing through a rapid channel, it is mixed together, and the coldest as well as the warmest part is brought into contact with the bed of the stream, the materials of which, acting as a point of rest, serve as a basis of

crystallization.

Peculiar mechanical effects are sometimes produced by alternations of thawing and freezing— as, for example, in the case of water pipes constructed of lead or other malleable metal. To render this plain, let us suppose a lead pipe one foot in length to be filled with water, and, after being hermetically sealed at each end, exposed to a low tem- perature; the expansion would merely stretch the pipe, the extension not being sufficient to burst it, and no continuation of cold or increase of its intensity would produce any further effect, as this would merely cause the ice to shrink; neither would thawing and refreezing produce any effect, since the water would merely return to its original volume, and the ice again expand to the same extent as before; but if the pipe communicated with a reservoir of water, so that when the thawing took place, the whole space, enlarged by the previous freez- ing, were again filled with water, a second freezing would produce another enlargement of its internal capacity, and a third thawing and freezing, under the same circumstances, would repeat the process, until at length the sides of the tube would give way.

Plants filled with sap and exposed to a low temperature, are variously affected, according to the character of the plant, the dura- tion of cold, and the season of the year at which it occurs. A sudden cold will tend to burst the cells. The velocity of the motion of the sap depends principally on the amount of evaporation from the leaves and stems, and this diminishes with temperature, all other things being the same; hence there is a certain degree of cold at which the sap ceases to flow, and the functions of the plant are sus- pended.

The different parts of the same plant are killed at different temperatures below thirty-two; the more succulent and tender suffer first, and the woody, or that in which the sap is better defended by non-conducting materials, last. A sudden fall of temperature, even if it be extreme, if of short duration, may not penetrate to the sap and produce freezing. It would also appear that the sap of different plants congeal at different temperatures, and it is highly probable

that other changes than those of a mechanical character are produced; but on this subject much research is required, and every intelligent farmer may add important materials to our stock of knowledge by carefully recording the observations he may make relative to the re- duction of temperature and its continuance by which certain plants

are destroyed.