Returning to our subject, let us consider the all-important question of range of climate. The effects of climate, unimportant to the stronger inhabitants of a country, but largely influencing the health and comfort of the majority, are chiefly felt through the changes that occur during the year. Now, we have seen that the line of mean annual temperature of England departs in a very marked manner from coincidence with a latitude-parallel; but we shall find the lines indicating the extreme temperatures of the year much more irregular; and the peculiarity of climate, which their conformation illustrates, much more important.
In Fig. 1 the isochimenal, or the line of equal winter heat, through London, is indicated by a strongly marked closed curve. Its form is remarkable. It passes nearly in a north and south direction, along the length of England and Scotland, approaches singularly near to Iceland, but turns sharply southwards and travels across the Atlantic in a direction which brings it to the American continent near Washington. Still approaching the tropics, it travels through the northern parts of Texas, where it reaches its greatest southerly range. Passing gradually northwards to the neighbourhood of the Aleutian Islands, it thence trends southwards again, passes through the Corea, traverses the Asiatic continent nearly on the latitude-parallel of Nankin; thence travelling slightly northwards, it crosses the southern part of the Caspian Sea, the Black Sea, and the north of Turkey, passing through Venice and Paris to London. On the continents the isochi-menal falls outside (that is, south of) the annual isotherm, while on the oceans the reverse holds. The projection of the isochimenal thus appears as an irregular oval, whose greatest length lies on the continents.
We see here, again, the indication of a tendency to form two curves, and thus of the presence of two poles of extreme winter cold in the northern hemisphere. The isochimenals of greatest cold hitherto traced in the two continents are shown by two broken curves in Fig. 1. The cold of the Asiatic curve is very much greater than that of the American, the former curve marking a winter cold of -40° Fahrenheit (72° below freezing), the latter a winter cold of -26° 5', only-if one may apply such an adverb to a cold of 58° 5' below freezing. Professor Nichol remarks that, 'if a polar projection were made of these regions for January, it would be found that the two coldest spaces of these continents form a continuous band passing across the pole of the earth.' I cannot but think that this is a mistake. I believe that if the isotherms traced, in part, in Fig. 1 could be completed, they would be found to form two ovals. The American oval would enclose the American pole of mean temperature, but very eccentrically, showing that the pole of extreme winter temperature lay westwards and southwards, probably near Victoria Land. The Asiatic oval would not probably enclose the Asiatic pole of mean temperature; and the position indicated for the Asiatic pole of extreme winter cold lies on or near the Arctic circle, where it is crossed by the river Lena. At the true pole of the earth the extreme winter cold is probably not nearly so intense as the cold at either of the points here indicated.
From the direction of the isochimenal through London, it is evident that the Eastern Counties and Kent experience the coldest winters of all places in the British Isles, while Cornwall and the south-westerly parts of Ireland enjoy the mildest winter climates. In fact, winter in Cornwall is not more severe than in Constantinople; and in south-west Ireland the winter is still milder, approaching, in this respect, to the winter climate of Teheran.
The contrast, when we turn to the isotheral of London, is remarkable. Instead of travelling nearly northwards, this curve passes in a south-westerly direction, reaching its greatest southerly range in the central part of the Atlantic Ocean; thence it travels with a northerly sweep through Nova Scotia and Canada, till it reaches its greatest northerly range near the Rocky Mountains.1 Thence it turns sharply southwards, crosses Vancouver's Island, sweeps nearly to latitude 45° in the central part of the Pacific, whence passing slightly northwards it crosses the southern part of Saghalien Island. Here it turns sharply northwards, crosses that very district of Siberia which, in Fig. 1, is occupied by the isochimenal of intensest winter cold, traverses Siberia, and passes near St. Petersburg, through Berlin and Amsterdam to London.
The relations thus presented by the isotheral of London are precisely the reverse of those exhibited by the isochimenal. The isotheral forms a closed irregular oval, whose greatest length lies on the two oceans: here it falls outside the line of mean annual heat, while on the continent it falls far within this line.
In another respect the isotheral presents a noteworthy contrast to the isochimenal. While the latter encloses an area largely exceeding the area enclosed by the mean annual line, the isotheral encloses an area noticeably smaller.2
A tendency to break up into two curves is exhibited in the isotheral, even more markedly than in the two other curves. But singularly enough, here, where one would expect more certain information of the existence of poles of cold, since so much more of the northern hemisphere can be traversed in summer than in winter, we have no satisfactory evidence. In fact, the irregular curve marked in near the pole in Fig. 2 is the most northerly isotheral yet determined. The temperature corresponding to this isotheral is 36° Fahrenheit, or four degrees above freezing. From a consideration of the form-variations of the isotherals as they travel northwards, I have been led to the opinion that there exist three poles of summer cold, and that these fall not very far from the positions indicated by the small dark circles in Fig. 2.