DISTRIBUTION OF TEMPERATURE IN THE ATMOSPHERE.

259. The distribution of heat over the surface of the globe is best represented by isothermal lines, or lines drawn through all places having the same mean temperature. The mean temperature of the warmest month in the northern hemisphere, July, is shown by the isothermal lines in Plate IV.; of the coldest month, January, in Plate V.; and of the year in Plate VI. These systems of lines are sometimes called isotherals, or lines of equal summer temperature; isochimenals, or lines of equal winter temperature; and isothermals, or lines of equal mean annual temperature.

260. In all the charts the part of the earth's surface where the highest temperature prevails forms an irregularly-shaped belt lying within the tropics, and comprised between the north and the south isothermals of 80°. On either side of this warm belt the temperature diminishes towards the poles; and the lines showing successively the gradual lowering of the temperature are, speaking in a loose sense, arranged parallel to the equator, thus illustrating the predominating influence of the sun as the source of terrestrial heat. While the decrease of temperature in advancing towards the poles corresponds, in a general way, to what may be called the solar climate, there are deviations brought about by disturbing causes too important to be overlooked. These disturbing causes are―(1), the currents of the sea; (2), large surfaces of water which are frozen over during part of the year; (3), the unequal distribution of land and water; (4), the prevailing winds; and (5), mountain-ranges.

261. The influence of an oceanic current on climate depends on the temperature of the place it leaves, and that of the place toward which it flows. Hence the great equatorial current flowing from east to west does not require to be considered here, inasmuch as the temperature remains the same throughout its course. It is those currents which convey the waters of the sea to high or to lower latitudes that require to be considered.

262. Gulf Stream.-Of these currents the most important as well as the most marked is the Gulf Stream, which, by conveying the warm waters of the south to the arctic regions, pushes the isothermals many degrees northward. The effect on the climate of western Europe during the year, and especially during winter, Plate V., receives striking illustration from the charts. If no more heat is received than is due to the position on the globe in respect of latitude, the mean winter temperature of Shetland would be only 3°, and that of London 17°. But chiefly owing to the heat given out by the Gulf Stream during winter, and carried to these places by the winds, their winter temperatures are respectively 39° and 37°-Shetland being thus benefited 36° and London 20° from their proximity to the warm waters of the Atlantic. In Iceland and the Norwegian coast, the increase thus accruing to the winter temperature is much greater. To these places the Atlantic may be conceived as a vast repository of heat, in which the warmth of the summer months, and that of more southern regions, are treasured up and reserved against the rigours of winter.

263. Winter Temperature of the British Islands.— The Gulf Stream leaves its impress unmistakably on the temperature of each of the months, as shown by the position of the monthly isothermals. In winter, fig. 18, the deviation from the normal position of the isothermals, from their east-and-west direction, is greatest. Indeed, as re

gards Great Britain, the lines are then at right angles to this normal direction, and lie north and south. In Ireland they seem to envelop the island with their folds, which increase in warmth from the centre of the island outward to the ocean. This points out clearly that the great source of heat from which the climate of Great Britain derives its warmth in winter is in the west; and that it is not to the winds alone, but also to the Atlantic Ocean, that we must look as the cause of almost the whole of this excess of temperature. For if the winds alone were the cause of the higher temperature, we should expect a different arrangement of the lines of equal temperature.

264. This peculiar distribution of the winter temperature of the British Islands has important bearings on the treatment

of diseases.

Since the temperature of the whole of the eastern slope of Great Britain is the same, it is evident that to those for whom a milder winter climate is required, a journey southward is followed by no practical advantage, unless directed to the west coast. And as the temperature on the west is uniform from Shetland to Wales, Scotland is as favourable to weak constitutions during winter as any part of England, except the south-west. The temperature on the south-west of England and Ireland being, however, 4° higher than the west of Scotland, the mildest climates, and therefore the most suitable resorts for invalids who require a mild climate, are to be found from the Isle of Wight westward, round the Cornish peninsula to the Bristol Channel, and from Carnsore Point in Ireland to Galway Bay.

265. A similar though much feebler current passes from the North Pacific, through or towards Behring Strait, and there accordingly the isothermals are pushed a little to the

northward. Part of this current returns by the west coast of America, depressing the temperature, especially the summer temperature, of these parts, Plate IV. The two great currents in the southern hemisphere flowing northward from colder to warmer latitudes-viz., Humboldt's, from the Antarctic Ocean to Lima by the coast of Peru, the other from the Cape of Good Hope along the west of Africa-lower the temperature, and thus drive the isothermals nearer the equator. It will be observed in Plate V., which represents the summer of the southern hemisphere, that Humboldt's current breaks the continuity of the equatorial belt of high temperature bounded by 80°. Again, the great equatorial current, after impinging on the east coast of Africa, turns southward, and by the warmth it imparts pushes the isothermals into higher latitudes along the east of Africa. And for the same reason the current flowing past the Brazil coast southward, raises the temperature in the east of that country.

266. In all cases the influence of these currents is most distinctly marked in January and July, the months of extreme temperatures. Thus the warm current of the Gulf Stream is most felt in January, when several of the isochimenals, or lines of mean winter temperature, are carried by it 1600 miles northwards of their normal position. Similarly the currents from the Antarctic Ocean being coldest in July, the isothermals are most deflected from their normal position during that month, the distance being about 1300 miles nearer the equator. About the most remarkable lowering of the isothermals occurs on the Labrador and Newfoundland coasts during May and June, being caused by the icebergs which then descend on these coasts through Davis Strait.

267. The great fresh-water lakes of North America-Lakes Superior, Huron, Erie, Michigan, Ontario, Bear Lake, &c.exercise an important influence on the climate of the interior of America. For in winter, America, with its either wholly or partially frozen lakes, is to a great extent an unbroken continental mass, and its winter climate may therefore be regarded as continental, except in a few limited localities

immediately adjoining the deepest lakes; whereas in summer its numerous large sheets of fresh water communicate to those parts many of the characteristic features of an insular summer climate. At Fort Brady, which is situated in the district where Lakes Superior, Michigan, and Erie nearly approach each other, the temperature of July is 64°.6; but at Fort Snelling, to the west, in Minnesota, and nearly in the same latitude, it is 73°.4, the difference being chiefly caused by the cooling effects of the inland seas which surround Fort Brady. On the other hand, the temperatures of these two places in January and February are the same. The curving round of the January isothermals of 32°, 20°, and 10° upon the regions surrounding the Baltic, is to some extent due to the freezing of the shallow, brackish waters of that sea during winter. Had the Baltic been deeper and salter, and not subject to freezing, the winter climate of places all round its coasts would have been much milder.

268. Since winds bring with them the temperature of the regions they have traversed, the equatorial current is a warm wind, and the polar current a cold wind. Also, since the temperature of the ocean is more uniform than that of the land, winds coming from the ocean do not cause such variations of temperature during the year as winds from a continent. As an atmosphere loaded with vapour obstructs both solar and terrestrial radiation, moist winds blowing from the ocean are accompanied by a mild temperature in winter, and a cool temperature in summer; and dry winds coming from continents, by cold winters and hot summers. Again, the equatorial current, losing heat as it proceeds in its course, is thereby brought nearer the point of saturation, and consequently becomes a moister wind; whereas the polar current, gaining heat in its progress towards the equator, becomes a drier wind. Hence the S. W. wind in Britain is a particularly moist wind, because it is both an oceanic and equatorial current; whereas the N. E. wind, on the contrary, is peculiarly dry and parching, because it is both a polar and continental current.