Such is the general climatic order as regards latitude; though, owing to the smaller extent of land, the decrease of heat as we depart from the equator is more rapid in the southern than in the northern hemisphere-so that, on a general average, the latter is about 3 warmer than the former.

243. Altitude is the next great modifier of climate, but owing to many correlative circumstances (prevalent winds, slope, proximity to the sea, &c.), its operation is not altogether uniform. As formerly stated, a decrease of 1o° Fahr. takes place in the lower regions of the atmosphere for every 300 or 350 feet of ascent; but at great heights and in extreme latitudes the decrease is more rapid. As it is, under every latitude the loftier mountains (Himalayas, Alps, Andes, &c.) are perpetually covered with snow; the higher plateaux (Mexico, Bolivia, Armenia, Tibet, &c.) are several degrees colder than the contiguous lowlands; and in temperate zones such table-lands experience much greater extremes of summer's heat and winter's cold than the surrounding districts-having, like Spain, Armenia, and Persia, summers of tropical heat and winters of almost polar severity. Another great cause of modification is the unequal reception and radiation of heat by land and water (par. 206), by which islands and sea-coasts are rendered cooler in summer and warmer in winter than inland tracts-creating what has been termed insular and continental climates. Britain, New Zealand, and Tasmania enjoy, in this respect, insular climates; Germany, central Russia, and Tartary, continental ones. Connected with this, and depending on the set of the trade-winds and ocean-currents, may be noticed the observation of Humboldt, that the continents and larger islands in the northern hemisphere are warmer on their western than on their eastern sides; while in the southern hemisphere the reverse holds good-the western being the colder and the eastern the warmer.

244. It would exceed the limits of a text-book to describe in detail the many causes concerned in the modification of climates; but besides those of latitude, altitude, and proximity to the sea, may be noticed the following:-1. The direction of mountainchains, which, by intercepting cold winds, renders the countries on one side warmer than those on the other; and in like manner, by intercepting moist winds, favours the production of rain on the windward slopes and droughts on the leeward or sheltered declivities. The eastern slopes of the Andes, for example, intercept the humid trade-winds from the Atlantic; their counter-slopes on the Pacific side are arid and rainless. 2. The general inclination or slope of a district, as this may lie to the heat of the morning and noonday sun, or be turned to the feebler rays of his afternoon and

evening declension. 3. Owing to their different capacities for heat, the relative masses and configuration of the land and water greatly affect the climatic peculiarities of a country; and this as moist and warm winds generally come from the sea, while cold and dry ones blow from the land—as the seaboard has a moist and cloudy sky that prevents radiation, while the interior has a serene sky that favours it and lowers the temperature-and as this radiation is greater in summer than in winter, and consequently interferes with the amount of heat directly received from the sun. 4. The prevailing winds of a region, as these may be cold or warm, dry or humid. The westerly winds of our own island are humid and warm-the easterly are cold and dry; hence the greater rainfall and mildness of our western coasts as compared with the eastern. 5. As with winds, so in like manner with the influence of oceanic currents. The Gulf Stream, by bringing warmth and moisture, mitigates the winter climate of western Europe; the Arctic current, on the other hand, tempers the summer climate of the eastern shores of North America. 6. The direction of river-plains and valleys, as they open out to favourable winds and ocean-currents. The westerly reception of the Rhine basin produces a finer climate than the easterly trend of the Danube, even though the latter be several degrees farther south; and the southerly slope of the valley of the Rhone enjoys amenities unknown in the westwardly-trending basins of the Loire and Garonne. 7. Cultivation has also a marked effect on the climate of a country—the felling of forests, draining of lakes and morasses, and the like, being all favourable to greater dryness, warmth, and general amenity.

Lines of Equal Heat, &c.

245. Under circumstances mentioned in the preceding paragraphs, it will readily be perceived that the belts of climate can by no means correspond with the parallels of latitude, but that it requires a long series of observations to determine the seasonal temperatures of summer and winter, and also a careful average of these to ascertain the mean yearly temperature of any given locality. If the surface of the globe had been all water, or all land of equal altitude, the parallels of latitude would have determined the lines of climate; but this not being the case, the daily temperature, the summer and winter temperatures, as well as the mean annual temperature of any two places in the same latitude, may differ very considerably. In this way the maximum

summer heat of an island may be several degrees below that of a continental country between the same parallels, while its winter temperature is many degrees higher. In this way, also, the mean annual temperature of some island or sea-coast may be equal to that of some inland district situated several degrees nearer the equator. A series of lines drawn through places having the same summer temperature show these variations at a glance, and are termed isotheral lines (Gr. isos, equal, and theros, summer), or lines of equal summer heat; those through places having the same winter temperature, isocheimōnal lines (Gr. isos, and cheimon, cheimonos, winter); and those connecting places of the same mean annual temperature, isothermal lines (Gr. isos, and therme, heat). The difference between the summer and winter temperatures may amount to 2°, 20°, 40°, or more degrees of Fahrenheit; but the isothermal lines show the mean amount of heat received throughout the year, and, of course, are the more correct indicators of the general climatic conditions of any given locality. In the accompanying Sketch-Map the isotherms are laid down for every ten degrees, and their bendings northward and southward (according to distribution of land and water, altitude, distance from sea, &c.) convey to the eye instructive proofs of the operating causes adverted to in the foregoing paragraphs.

[In amplification of the preceding remarks on Climatology, the following passage from Mrs Somerville may be read by the student with advantage:-"Places having the same mean annual temperature often differ materially in climate. In some the winters are mild and the summers cool, whereas in others the extremes of heat and cold prevail. England is an example of the first; Quebec, St Petersburg, and the arctic regions are instances of the second. The solar heat penetrates more abundantly and deeper into the sea than into the land; in winter it preserves a considerable portion of that which it receives in summer, and from its saltness does not freeze so soon as fresh water; hence the ocean is not liable to the same changes of temperature as the land, and by imparting its heat to the winds it diminishes the severity of the climate on the coasts and in islands, which are never subject to such extremes of temperature as are experienced in the interior of continents. The difference between the influence of sea and land is strikingly exemplified in the high latitudes of the two hemispheres. In consequence of the unbounded extent of the ocean in the south, the air is so mild and moist that a rich vegetation covers the ground, while in the corresponding latitudes in the north the country is barren from the excess of land towards the Polar Ocean, which renders the air dry and cold. A superabundance of land in the equatorial regions, on the contrary, raises the temperature, while the sea tempers it.

"Professor Dove has shown from a comparison of observations that northern and central Asia have what may be termed a true continental climate both in summer and in winter-that is to say, a hot summer and cold winter; that Europe has a true insular or sea climate in both seasons, the summers being cool and the winters mild; and that in North America the climate is inclined to be continental in winter and insular in summer. The extremes of temperature in the year are greater in central Asia than in North America, and greater in North America than in Europe,

and that difference increases everywhere with the latitude. In Guiana, within the tropics, the difference between the hottest and coldest months in the year is only 2°.2 Fahr., in the temperate zone it is about 60°, and at Yakutsk, in Siberia, 114°.4. Even in places which have the same latitude, as in northern Asia compared with others in Europe or North America, the difference is very great. At Quebec the summers are as warm as those in Paris, and grapes sometimes ripen in the open air, yet the winters are as severe as those in St Petersburg. In short, lines drawn on a map through places having the same mean summer or winter temperature are neither parallel to one another, nor to the isothermal or geothermal lines, and they differ still more from the parallels of latitude."]

NOTE, RECAPITULATORY AND EXPLANATORY,

'In the two preceding chapters attention has been directed to the Climatology of the globe—that is, to those weather-conditions on which its vegetable and animal life are so intimately dependent. The main medium of climate being the Atmosphere, it was necessary to advert to its nature and composition as an integral portion of our planet. As an aërial fluid, it consists of an admixture of 79 parts nitrogen and 21 oxygen, together with a small but variable proportion of carbonic acid gas, traces of ammonia, &c., and always holding in suspension less or more of aqueous vapour in a visible or invisible form. Constituted as plants and animals are, this composition is indispensable to their existence—the former assimilating carbonic acid and exhaling oxygen, the latter, in counterbalance, exhaling carbonic acid gas and consuming oxygen. Light and invisible as this aërial envelope may appear, it exerts a pressure on the earth's surface, at the sea-level, of about 14 lb. avoirdupois to the square inch—a pressure which is balanced by a column of mercury 30 inches in height; hence the barometric column of the meteorologist, and the rise and fall of the barometer according to changes in the weight of the atmosphere. As an elastic or compressible medium, its lower strata are denser or heavier than those at great elevations; and as its capacity for heat and moisture decreases with its rarity or attenuation, the higher regions of the air are colder and drier than those at a lower elevation.

As the medium through which the light and heat of the sun are conveyed to the terraqueous surface, the atmospherepartly owing to the varying inclination of the sun's rays, and partly to the unequal reception and radiation of heat by land and water-becomes variously heated in its different regions, and