Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/15

Rh below that of the mean annual temperature of the locality. From this follow two important consequences, viz. (1) during each winter no inconsiderable portion of the cold produced by terrestrial radiation is conveyed away from the surface to the depths of the lake, where it therefore no longer exercises any influence whatever on the atmosphere or on the climate of the district in lowering the tem perature ; and (2) this annual accession of cold at these depths is wholly counteracted by the internal heat of the earth. In corroboration of this view it may be pointed out that the water of the Rhone as it issues from Lake Geneva is 3 7 higher than that of the air at Geneva. Thus, the influence of lakes which do not freeze over is to mitigate in some degree the cold of winter over the district where they are situated. This is well illustrated ori a large scale by the winter temperature of the lake region of North America. The influence of the sea is exactly akin to that of lakes. Over the surface of the ground slanting to the sea-shore the cold currents generated by radiation flow down to the sea, and the surface-water being thereby cooled sinks to lower depths. In the same manner no inconsiderable portion of the cold produced by radiation in all latitudes over the surface of the ocean and land adjoining is conveyed from the surface to greater depths. The enormous extent to which this transference goes -on is evinced by the great physical fact disclosed to us in recent years by deep sea observations of temperature, viz., that the whole of the depths of the sea is filled with water at or closely approaching to the freezing point of fresh water, which in the tropical regions is from 40 to 50 lower than the temperature of the surface. The with drawal from the earth s surface in high latitudes of such an enormous accumulation of ice-cold water to the depths of the sea of tropical and subtropical regions, rendered possible by the present disposition of land and water over .the globe, doubtless results in an amelioration to some extent of the climate of the whole globe, so far as that may be brought about by a higher surface temperature in polar and temperate regions. Oceanic climates are the most equable of all climates, showing for the same latitudes the least differences between the mean temperatures of the different hours of the day and the different months of the year, and being at all times the least subject to violent changes of temperature. So far as man is concerned, oceanic climates are only to be met with on board ship. The hygienic value of these climates in the treatment of certain classes of chest and other complaints is very great, and doubtless when better understood in their curative effects they will be more largely taken advantage of. It is, for instance, believed by many well qualified to form an opinion that they afford absolute, or all but absolute, immunity from colds, which are so often the precursors of serious complicated dis orders. The nearest approach to such climates on land is on very small islands such as Monach, which is situated about seven miles to westward of the Hebrides, in the full sweep of the westerly winds of the Atlantic which there prevail. The mean January temperature of this island, which is nearly in the latitude of Inverness, is 43 - 4, being 1 8 higher than the mean of January at Ventnor, Isle of Wight, 8 higher than that of Jersey and Guernsey, and almost as high as that of Truro. Again, Stornoway, being situated on the east coast of Lewis on the Minch, an inland arm of the Atlantic, has thus a less truly insular position than Monach. Its climate is therefore much less insular, and accordingly its mean temperature in January is 38 7, or 4-7 lower than that of Monach. From its position near the Moray Firth, on the east of Scotland, Culloden occupies a position still less insular ; hence its January temperature is only 37 l, being l-6 less than that of Stornoway, and 6 - 3 less than that of Monach. On the other hand, the mean temperature of July is 55-0 at Monach, 57 8 at Cullodeu, GT O at Guernsey, and G2 G at Ventnor. Thus the conditions of temperature at these stations are completely reversed in summer, for while in January Monach is l - 8 wanner than Ventuor, in summer it is 7 G colder. Since the prevailing winds in the British Isles are westerly, places on the east coast are less truly insular than are places similarly situated on the west, whence it follows that the winter and summer climates of the east coast approach more nearly the character of inland climates than do those of the west. The facts of the temperature at such places as Monach in Scotland and Valentia in Ireland disclose the existence of an all but purely oceanic climate along the coasts, particular^ of the west, so distinct and decided, and extending inland so short a distance, that it would be impossible to represent it on any map of land isothermals of ordinary size. The only way in which it can be graphically represented is by drawing on the same map the isothermals of the sea for the same months, as Petermann has done on his chart of the North Atlantic and continents adjoining. Such maps best lead to a knowledge of the true character of our seaside climates. Though it is Impossible to overestimate the climatological importance of seaside climates, as evinced by their curative effects on man, and their extraordinary influence on the distribution of animal and vegetable life, it must be con fessed that we are yet only on the threshold of a rational inquiry into their true character. Undoubtedly the first step in this large inquiry is the establishing of a string of about six stations at various distances from a point close to high-water mark to about two miles inland, at which observations at different hours of the day would be made, particularly at 9 A.M. and 3 and 9 P.M., of the pressure, tem perature, humidity, movements, and chemistry of the air. Our large towns have climates of a peculiar character, which may be said to consist chiefly in certain disturbances in the diurnal and seasonal distribution of the temperature, an excess of carbonic acid, a deficiency of ozone, and the presence of noxious impurities. Systematic inquiries into the condition and composition of the air of our large towns have been instituted this year (1876) in Paris and Glasgow, in which the ozone, ammonia, nitric acid, and germs present in different districts of these cities are regularly observed. There yet remain to be devised some means of making truly comparable thermometric and hygrometric observa tions in different localities, including the more densely- peopled districts, for the investigation of what we may call the artificial climates peculiar to each district. While such an inquiry, at least in its earlier stages, must necessarily be regarded as a purely scientific one, it may fairly be expected to lead sooner or later to a knowledge of the causes which determine the course of many epidemics- why, for instance, diphtheria is more frequent and more fatal in the new than in the old town of Edinburgh, and why in some parts of Leicester diarrhoea is unknown as a fatal disease, while in other parts of the same town it rages every summer as a terrible pestilence among infants and ultimately suggest the means by which they may be stamped out when they make their appearance. It has been already pointed out (see ) that prevailing winds are the simple result of the relative distri bution of atmospheric pressure, their direction and : being the flow of the air from a region of higher towards a region of lower pressure, or from where there is a surplus where there is a deficiency of air. Since climate is pract! cally determined by the temperature and moisture of t wr, and since these are dependent on the prevailing winds wmci 