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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 below 7200 feet is invariably as low as 39°, and that by far the larger portion of the water of the ocean remains constantly at this low temperature? In the first place, it must be kept in mind that the whole water of the seas over the globe is one body, and on account of its fluidity a free communication is kept up among its different parts; and, in the second place, since sea-water contracts, and consequently gains in density, as it is cooled until it freezes, the cold water tends everywhere to flow towards and settle in the depths of the ocean.
From this it follows that the water of the sea will, below a certain depth, fall to one uniform temperature, and that temperature just as cold as the surface temperatures of the sea over the whole globe can reduce it to. It is remarkable that this uniform temperature of 39° is the temperature at which pure water attains its maximum density. It may, however, be added, that this agreement is no more than a mere coincidence, there being no connection between the two facts.
226. Surface Temperature of the Sea.This is one of the most important problems of meteorology and physical geography, but it has as yet been worked out only so far as to obtain no more than the very rudest approximation to a solution. The largest collection of facts bearing on the subject is in possession of the Board of Trade, consisting of about 550,000 different observations. In addition to these it is estimated that upwards of a million are still required before sufficient materials are obtained for solving the problem of the temperature of the sea. The British Government has taken steps to obtain the observations still desiderated, and to digest, tabulate, chart, and publish the results.
227. Temperature of the Sea round Scotland.—The part of the sea whose temperature is best known, is that portion surrounding the Scottish coast, from which, owing to the labours of the Scottish Meteorological Society, we have systematic observations during the last ten years. From these observations we learn that the mean annual temperature at the mouth of Loch Fyne is 499.0, west of Oban 48°.7, at Harris in Lewis 48°.9, Orkney 480.8, Shetland 48o.4, and the mouth of the Firth of Forth 47°.8. Hence the Atlantic is 19.0 warmer than the North Sea off the east coast of Scotland. The sea is also 1°.0 warmer than the air, and among the islands in the north it is 3.0 warmer. But it is during winter that the difference between the temperature of the Atlantic and the North Sea is most apparent. The mean temperature of the Atlantic in July is 54o.4, and in January 44.7; whilst the North Sea is 55°.5 in July, and 40°.8 in January. And if extreme temperatures be considered, the advantage in favour of the Atlantic is greatly increased, for the lowest temperature to which the Atlantic has yet been observed to fall is 39o.1, whereas at the extremity of Trinity Chain Pier in the Firth of Forth, the temperature of the sea fell to 339.7 in February 1865. These are the temperatures of deep water ; but in shallow water much lower temperatures occur.
Richard Adie has observed the temperature of the sea in the shallow estuary of the Mersey as low as 29°; and during the severe frost of Christmas. 1860, the Moray Firth was frozen to a considerable depth a mile seaward.
228. Temperature of the Sea in different parts of the Globe. -- Among the papers left in an unfinished state by Admiral Fitzroy, was one on the temperature of the sea. has since been published, and is accompanied by a chart, on which the surface of the globe is divided into spaces ranging between 80° N. lat., and 70° S. lat., and bounded by each tenth meridian and tenth parallel. Those spaces, in themselves of unequal areas, and of different shapes, are named ten-degree squares, because of their uniformly rectangular appearance in the charts drawn on Mercator's projection. The mean temperature is printed on these ten-degree squares. Partly owing to the fewness of the observations made in many of the squares, and partly to the method adopted in reducing the observations, this chart of sea temperatures cannot be considered otherwise than as a rude approximation to the real temperatures of many of the squares.
In the square including Great Britain, the mean temperature of the sea is
stated to be 550.8, which is, without a doubt, at least 3° or 4° too high ; and in the adjoining square to the east, including the North Sea, the temperature of that sea is given as 50°.9, perhaps also a little too high. In spite, however, of this serious drawback, it is, and will be for some years to come, our best source of information regarding the temperature of the sea taken as a whole. On this ground we shall give some account of the temperatures as there laid down.
229. Owing to the small change of temperature within the tropics, the temperature of the sea in these parts is probably very near the truth. It is generally from 80° to 830.5.
230. Mean Temperatures of the different Oceans. --Between the limits of 50° north and 50° south, the mean temperature of the North Atlantic is 71°.6, and that of the South Atlantic 66°.7. The South Atlantic is thus 5o colder, and this difference is nearly uniform for corresponding parallels of latitude. Similarly the mean temperature of the North Pacific is 699.9, and that of the South Pacific 679.7. Hence this ocean is colder south of the equator, but the difference is not so great as in the Atlantic. Of the three oceans south of the equator --the Atlantic, the Indian, and the Pacific—the Atlantic is the coldest, being 66.7; the Indian is the warmest, 699.3; and the Pacific is between the two, 67o.7. On the other hand, the North Atlantic is 10.7 warmer than the North Pacific.
231. The mean temperature of the western half of the Mediterranean Sea is about 65°, and that of the eastern half, from 3° to 6° warmer; while that of the Black Sea is only 56°.8. On the other hand, the mean temperature of the Red Sea north of 20° lat. is 77o.4, but south of that parallel it is 810.5. The great differences between the temperatures of these three seas cannot but exercise a powerful influence on the climates of Palestine, Asia Minor, and adjacent countries.
232. The highest temperature anywhere yet observed is 94° in the Red Sea, near Aden; the highest temperatures elsewhere are 91°, near Siam, and 89° and 88° in several places in the Indian Ocean near the equator.