K70l

- erui mai- Wa K te

uäle

lr h ordel tich anedt N- d Ual ranos rthe

METEOROLOGVY. 505

animal is frozen entirely through, and that every vital act is sus- pended? To what degree can a like result be produced on warm- blooded animals, and how far can the state of hibernation be pro- longed without death to the individual? Will it ever be possible, in the case of the higher mammalia, to so maintain the unstable equi- librium as to prevent decay, and, at the same time, to preserve, as it were, in a latent state the vivifying principle? Though investiga- tions on this point would be interesting, we can scarcely hope to realize from them one of the fancies of Dr. Franklin, that of sending representatives of one age down to another to keep alive more actively the sympathies of the present with the past?

The depth to which ground is frozen in some places, from year to year, is also an indication of the severity of the seasons; the effect of cold, however, will penetrate very differently in dry or moist soil; in the first it will depend entirely on the conducting power of the mate- rial, and in the second, it will also depend upon the amount of water to be congealed. The conducting capacity being the same, the depth to which the given degree of cold will penetrate will be much greater in dry than in wet soil, on account, of the great amount of latent heat given off by the water before it is solidified. In dry conducting soil, the propagation of cold downwards may continue some time after the surface of the ground has become considerably heated.

In a conducting body, all parts tend to an equilibrium of tempera tureé. If the upper end of a vertical iron bar be heated, and then removed from the source of heat, it gradually becomes cooled, while the other parts increase in temperature, until gradually an equilibrium is established; conversely, if we cool the upper end of the bar, it will take heat from the next lower part; and this from the next, and so on, until the cooling reaches the extreme end, which will be cooled last. If, before the cooling has reached the lower end, we heat the upper part, the next below will be heated, and so on, pro- ceeding downwards; thus waves, as it were, of heat and cold may be sent through the length of the bar, becoming less and less in in- tensity as they descend. In this way explanations have been given of the phenomenon of caverns colder in summer and warmer in winter— the cold wave due to a lower temperature requiring six months to reach the point of observation.

The freezing of the ground in certain soils is hurtful to vegetation; the frozen stratum, expanding irregularly from pelow, heaves up the surface, and frequently loosens or breaks the roots of the plant. A covering of snow is a protection, since this substance, from its floc- culent nature and the air entangled in it, is a bad conductor of heat. As a general rule, during cold weather a thermometer in air on the snow will exhibit a lower temperature than one under the same material at the surface of the ground. This effect, however, is not entirely due to the screening influence of the covering, but in part to the fact that the intense rays of the heat of the sun, as well as those of the light of the same body, penetrate the crystals of the snow as they do the glass covering of a hot-house, and, absorbed by the dark ground beneath, elevate the temperature. For the same reason in