The most important agents in the distribution of heat are marine and aerial currents. The former only are properly within the scope of this work, and we must therefore refer the student desirous of investigating the climatical influence of the latter to other works on Physical Geography, in which the subject is fully discussed. 1 That the great ocean currents do exert a perceptible influence on the climate of countries to wards which they flow is evident. Take the North Atlantic, for instance. Its eastern shores are bathed in warm water, most probably the prolongation of the Gulf Stream. The result is that, during the severest winters, the British seas are perfectly open, and the whole coast to Norway is clad with a luxuriant vegetation. Its western shores are washed by the cold, iceberg-laden Arctic current—with the result that, during the winter, the ports north of Halifax are ice-bound ; and the cold, sterile coasts of Labrador and New foundland contrast strangely with the north of France and the Emerald Isle. One might bathe off the North Cape of Norway, in 71° N. lat., in water as warm as that in the har bour of New York, on the opposite side of the Atlantic, in 40° N. lat., two thousand miles further south. ' Geikie re marks that the harbour of St. John's, Newfoundland, although two degrees further south than Liverpool, has been frozen over in June.
The Japan current and westerly drift towards the western shores of North America, are, in the Pacific, what the Gulf Stream and north-easterly drift are in the Atlantic. While the Columbian and Californian coasts are always open, the Sea of Okhotsk is frozen over during the winter months.
Whether the winds or ocean currents are the more important agents in the distribution of heat, is as yet a moot point ; but the climatical influence of currents alone must be very great. The heat conveyed into the North Atlantic by the Gulf Stream has been estimated at one-fourth the total solar heat received by that ocean. The stoppage of this cur rent alone would cause a loss of one-fourth the heat which renders North-western Europe habitable. Were the Gulf Stream stopped, icy-cold currents from the Arctic would sweep past Norway, and, enveloping the British Isles in their chilly grasp, would cumber our coasts with ice even in summer. The Black current of the Pacific is almost as important as the Gulf Stream. Thermal currents also set southward along the eastern shores of South America and South Africa, but these are inferior in size and as heat distributors compared with the vast northerly currents. The additional heat con veyed by the latter results in the Northern Hemisphere having a higher mean temperature than the Southern. These conditions point to the enormous climatical influence of ther mal oceanic currents. Owing to the earth's spherical form, a
superabundance of heat is received at the equator ; whilst in high latitudes far too little is received to make the earth habitable for mankind. Oceanic currents, as we have seen, modify this state of things, by transferring heat from the Torrid Zone to the Temperate and Frigid Zones. "Stop the oceanic currents, and the world is, in high regions, unin habitable." The difference as regards heat, owing to the proximity of the sea, affords us a basis for a classification of climates. The equable climates of countries near the sea directly subject to its beneficent influence may be called maritime or insular; while the excessive heat and intense cold of inland countries are the characteristics of an extreme or continental climate. The climatical influence of the sea is perhaps most strikingly shown by contrasting the average temperatures of maritime and inland countries in the same latitude. Edinburgh and Moscow have nearly the same latitude : but while the average summer and winter temperatures of the former is 57° and 38°, those of the latter are and respectively—thus showing a mean annual range at Edinburgh of only 19°, but at Moscow of 49°. The excessive heat, and still more intense cold, of Cen tral Asia, prove indirectly the indissoluble connexion between equability of temperature and proximity to the sea. Hue tells us that "in the deserts of Tartary, and especially in the coun try of the Khalkas, the cold is so terrible, that during a great part of the winter the mercury freezes in the thermometer ; and often when the earth is covered with snow, and the north west wind begins to blow, it drives the avalanche before it, till the whole plain looks like a white stormy ocean." The most perfect examples of insular climates are to be found among the islands of the South Sea, the range in many groups scarcely exceeding five or six degrees throughout the year. New Zealand and Tasmania, in the Southern Hemisphere, and Great Britain, in the Northern, have com paratively uniform temperature—varying only 20° on an average in the year. As an example of a strictly continental climate, we may take Central Asia, where—in the same lati tude as Great Britain—the winter temperature sinks 32° below freezing point, rising in summer to 70°, the range being thus fully 70°. But even this is exceeded, for at Yakutsk, in Siberia, the winter temperature is – 40°, while that of the summer is 62° ; the range thus amounting to 102°. The influence of the sea in the distribution of climate is graphically shown at a glance in charts of isothermal lines, the diffused warmth of the thermal currents bending the lines towards the poles, while the cold polar streams deflect them towards the equator.