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position. Radiation will proceed over the whole at the same rate, but the effects of radiation will not be felt everywhere in the same degree and intensity. For as the air in contact with the declivities of hills and rising ground becomes cooled by contact with the cold surface of the ground, it acquires greater density and weight, and consequently slips down the slopes and accumulates in the low-lying ground at their base. Hence places situated on rising ground are never exposed to the full intensity of frosts; and the higher they are, relatively to the surrounding ground, the less are they exposed, because they are protected by their elevation, which provides, as it were, an escape for the cold almost as fast as it is produced. On the contrary, valleys more or less environed by hills or eminences, not only retain their own cold of radiation, but also serve as reservoirs for the cold of neighbouring heights. Hence low-lying places are peculiarly exposed to intense cold. Plains and table-lands are simply affected by their own radiation.
214. This explains why vapour becomes visible so frequently in low places, whilst adjoining eminences are clear; and the same fact instinct has made known to cattle and sheep, which generally prefer to rest during night on knolls and other eminences. Along most of the watercourses of Great Britain, during the memorable frost of Christmas 1860, laurels, araucarias, and other trees growing below a certain height were destroyed, but above that height they escaped ; thus attesting, by unmistakable proof, to the great and rapid increase of the temperature with the height above the lower parts of the valleys.
215. Distribution of Cold in Mountainous Countries during Winter.-From the above remarks, it is evident that the distribution of temperature over the surface of a mountainous country during calm weather in winter, will be regulated by a different law than that of height above the sea. For the illustration of this point, it is fortunate that sixty-nine meteorological stations were established in Switzerland in 1863 ; and as one of the firstfruits of this Society, a paper has been lately published by Professor E. Plantamour, on the Distribution of Temperature on the surface of Switzerland during the winter of 1863-64. Whenever the soil is colder than the air above it, the superficial layers become cold by contact, as already explained, and a system of descending air-currents sets in over the whole face of the country. The direction and intensity of these descending currents are modified by the irregularities of the ground, and, like currents of water, they tend to converge and become united in the
gorges and ravines, down which they flow like rivers in their beds. These currents give rise to counter-currents flowing over them to supply their place.
216. When the station is on the top of a mountain, as the Righi, the counter-current comes from a great height above the ground, and being therefore warmer, the temperature of such stations is comparatively high. At places situated on the sides of mountains, the influence of the counter - current tends to raise the temperature, though in a less degree than at the top, on account of the descending current from the heights above mixing with it. The Swiss villages being generally built on eminences rising out of the side of the mountains, and bounded on both sides by gorges and ravines, are admirably protected from the cold of winter. For the descending currents flow aside into the gorges, and the counter-currents are constantly supplying warmer air from the upper regions of the atmosphere.
217. Though the space occupied by the current of cold air in the bottom of a valley is of greater extent than the bed of a river, it is nevertheless limited, and on all occasions tolerably defined, so that on rising above it in ascending the slope, an increase of temperature is readily perceptible. The gradual narrowing or contracting of a valley has a very appreciable influence in lowering the temperature; for the valley is thus transformed into a basin almost closed, into which cold currents of air descend from all sides. On such occasions, a cold wind rushes impetuously down the narrow gorge, which serves as an outlet to the basin; and it is on this principle that many
of the sudden gusts and breezes peculiar to mountain districts, such as vent du Mont Blanc, are to be explained. When the basin is a deep lake, the cold which is poured down on its surface, having cooled the surface-water, is thereby conveyed to greater depth, and has therefore scarcely any effect in lowering the temperature of the air resting over the lake. Hence lakes are a source of heat during winter, and places situated at their outlet are not exposed to such gusts of cold wind as those referred to above.
218. Influence of Forests.—The temperature is found to be warmer at the base of a mountain, and up its sides, when the slopes above are covered with trees. The beneficial influence of forests is exerted in two ways—viz., in the diminished radiation from the surface protected by the trees, and in the obstacle they oppose to the descending currents of cold air.
219. On the contrary, the cold of winter is more severely felt in those localities where the slopes above are destitute of vegetation, and consist only of bare soil and rocks, or of snow.
220. This peculiar distribution of the temperature only takes place during comparatively calm weather; during windy and stormy weather the law of the decrease of temperature with the height takes effect.
221. Situations which afford the best protection against the Cold of Winter.-In countries such as Great Britain, and, indeed, in most temperate countries, the majority of the deaths which occur are occasioned, or at least hastened, by low temperatures. In the tables of mortality and temperature, published weekly by the Registrars-General for England and Scotland, we have constant proof of this statement. For when, during the cold months of the year, the temperature happens to fall a few degrees, the death-rate at once rises to a height proportioned to the depression of the temperature. It is thus a matter of most vital importance, especially to invalids, to know the local situations which afford the best protection from low temperatures. From what has been already said regarding the increase of temperature with the height, it is evident that mere local situation may, during periods of great cold, have the effect of maintaining the temperature many degrees higher than what prevails at lower situations near at hand—a difference which will frequently assuage suffering, and on certain occasions save life. The advantage will of course be the greater if the sleeping-apartments be in the higher flats of the house. The dwellings most protected against severe cold are those situated on a gentle acclivity, a little above the plain or valley from which it rises, having a southern exposure, and the ground behind planted with trees. It is chiefly on account of these natural advantages that Bridge of Allan owes its popularity as a winter resort for invalids. It may not be irrelevant here to draw attention more particularly to the degrees of advantage held out by different sites in the same town or village—the low-lying being the coldest, and those on slopes and sheltered by trees being the mildest.
THE DISTRIBUTION OF TERRESTRIAL TEMPERATURE.
222. The distribution of terrestrial temperature may be conveniently treated of under three heads-viz., the temperature of the sea, of the land, and of the air.
THE TEMPERATURE OF THE SEA.
223. The most striking fact regarding the temperature of the sea, as ascertained by soundings, is that below a certain depth, dependent on the latitude, an invariable temperature of about 39° prevails. The depth at which this temperature is met with at the equator is about 7200 feet. On receding from the equator it becomes less, until about latitude 56° it reaches the surface, unless where superficial currents push it into higher or lower latitudes. From 56° lat. towards the poles this uniform temperature descends, till in 70° lat. it is 4500 feet below the surface.
224. Thus, then, the surface of the ocean is divided into three great regions--one surrounding each pole, where the temperature is below 39°, falling at the coldest parts to the freezing-point of sea-water; and the third, the zone between these two, the temperature of which is everywhere higher than 39°, rising at some places within the tropics to an annual mean of 85°.
225. How comes it that in the warmest parts of the globe, within the tropics, the temperature of the sea at all depths