1 It is noteworthy that the minimum distance of the isotheral from the North Pole here attained is exactly equal to the minimum distance of the isochimenal from the equator.
2 Here an important advantage of the isographic projection is exhibited. The relation pointed out is altogether obliterated in Mercator's projection, and could only be roughly inferred from any but an isographic projection.
From the direction of the isotheral line through London, it is evident that along the south-eastern coast of England the heat of summer is greater than in any other part of the British Isles. On the other hand, the northern parts of Scotland, which we have seen enjoy a winter climate fully as warm as that of London, have a much cooler summer climate. The southwestern parts of Ireland exhibit a change even more remarkable For whereas the winter climate in these parts is the same as that of Persia, the summer climate is the same as that of the very portion of Siberia in which (most probably) the greatest cold ever observable in our northern hemisphere is experienced in winter. The summer of the Orkney Islands, again, is no warmer than that of the southern parts of Iceland.
It appears, then, that the inhabitants of England enjoy three notable advantages as respects climate. First, a higher mean annual temperature than that of any other country so far from the equator; secondly, a moderate degree of cold in winter; and lastly, a moderate degree of heat in summer. The last two advantages resolve themselves into one, viz., small range 6f temperature throughout the year. Our range of climate is from about 36° in winter to 62 1/2° in summer, or in all, 26 1/2° Fahrenheit. Compare with this the climate of the country near Lake Winnipeg, with a winter cold of 4° below zero, and a summer heat scarcely inferior to that of London; so that the range of climate is no less than 65°. Yet more remarkable is the variation of climate in parts of Siberia, near Yakutsk ; here the range is from - 40° in winter to 62° in summer-a variation of 102°, or four times the variation of our London climate. Other parts of the British Isles have, however, a yet smaller range even than that of London. Thus in the south-western parts of Ireland, and in the Orkney Isles, the variation is less than 19°.
Nor is it difficult to assign sufficient reasons for the mildness of the British climate - for our warm winters and cold summers. It will appear, on examination, that nearly all the constant causes affecting the temperature of a climate operate to raise the mean temperature of our year, while, of variable causes those which tend to generate increased heat operate in winter, while those which have a contrary effect operate in summer.
Humboldt enumerates among the causes tending to exalt temperature the following non-variables: - The vicinity of a west coast in the northern temperate zone; the configuration of a country cut up by numerous deep bays and far-penetrating arms of the sea; the right position of a portion of the dry land, i.e. its relation to an ocean free of ice, extending beyond the polar circle or to a continent of considerable extent which lies beyond the same meridional lines under the equator, or at least in part within the tropics; the rarity of swamps which continue covered with ice through the spring, or even into summer; the absence of forests on a dry, sandy soil; and the neighbourhood of an ocean-current of a higher temperature than that of the surrounding sea.
All these causes, it will be observed - except the neighbourhood of a tropical continent on the same meridian - tend to increase the mean heat of the climate in England. The great Gulf Stream probably exercises a more important influence than any of the others. Its position is indicated in Figs. 1 and 2. Humboldt attaches a high importance to the presence of a tropical continent on the same meridian; and he considers that the climate of Europe is warmer than that of Asia, because Africa, with its extensive heat-radiating deserts, lies to the south of Europe, while the Indian Ocean lies to the south of Asia. There are objections, however, to the reasoning he adopts. In the first place, if the heat-radiating power of a continent really influenced the country lying to the north, it should tend to lower rather than raise the temperature, for the ascending currents of air would strengthen the currents of colder air pouring in from the north, and these currents -on Humboldt's assumption that the country directly to the north is that affected - would lower the mean annual temperature. It would only be exceptionally that the warmer returning currents would descend, and thus exalt the temperature. It seems clear, however, that Asia is the continent chiefly affected by the heat-radiating power of Africa; since the cold currents from the north travel eastwards, while the warm return-current has a westerly motion. We should thus attribute the milder climate of Europe rather to the influence of the tropical parts of the Atlantic Ocean, than to the cause assigned by Humboldt, and we should invert the effects he attributes to oceans and continents respectively. With this change - somewhat a bold one, I confess1 - we may say that all the non-variable causes tending to exalt temperature operate in England's favour.
The constant causes tending to lower temperature are simply the converse of those above considered.
1 Not unsupported, however, by good authority. Thus Professor Nichol, speaking of the climate of Europe, writes: 'The air that rises in Africa blows rather over Asia than Europe. The cradle of our winds is not in Sahara but in America.' Again, Kaemtz notices, that if the effects of oceans and continents were those assigned by Humboldt, we should find in the western parts of America a colder climate than in the eastern parts; the reverse, however, is the case.
Of variable causes increasing temperature, the principal are a serene sky in summer, and a cloudy sky in winter. It may appear, at first sight, paradoxical to assign opposite effects to a cloudy sky. It must be remembered, however, that clouds considered with reference to temperature, have two functions: they partially prevent the access of heat to the earth, and they partially prevent the escape of heat from the earth. Now, in summer the first-named influence is more important than the second: the days are longer than the nights; that is, the earth is receiving heat during the greater part of the time in summer. A cause, therefore, which affects the receipt of heat is more important than a cause affecting the escape of heat. On the other hand, in winter the nights are shorter than the days, and the influence of clouds in preventing the escape of heat becomes more important than their effect on the receipt of heat.1 In fact, we may compare the influence of clouds to the effects of certain kinds of clothing; flannel, for instance, is as suitable an article of dress for the cricketer as for the skater.