so often thoughtlessly considered as mere hindrances, and fortuitous incumbrances, are, in reality, most pregnant causes for gratitude to Him who "by his strength setteth fast the mountains."

The subject of mountains is peculiarly appropriate to the object of this work. The emigrant and traveller may witness them on a large scale. The continent of New Holland, for example, is marked on its eastern coast by a chain of mountains 1500 miles long, which has generally a meridional direction, and never deviates much from the coast. Their average height, however, is only from 2400 to 4700 feet; and the loftiest of them, Mount Kosciusko, does not exceed 6500 feet. Their character is peculiarly rugged and savage; in some cases round at the top, and crowned with forests, but generally, though wooded on their flanks, terminating in bare toothshaped peaks, and flat crests of granite or porphyry, mingled with patches of snow. In New Zealand, chains of lofty mountains pass through the islands, rising in New Ulster 14,000 feet above the stormy ocean around. They exhibit on the grandest scale all the alpine characters, with the addition of some active volcanoes.

Such phenomena are worthy of notice, inasmuch as the mountain-ridges, the spreading lakes, the grassy steppes, the extended prairies, and even the voiceless deserts, surrounded by wooded regions, or by sea-skirting coasts-all these, in their several modes, impress a peculiar character on congregated communities of mankind.

Lofty chains of mountains capped with snow

do indeed interrupt communication, and interfere with traffic; but a mixture of less elevated mountains and hills lying apart, with low and level lands, and fertile valleys, presents a most happy interchange, occasions variety in the conditions of the atmosphere, and in the products of the vegetable kingdom, and begets wants, to satisfy which activity is aroused, and the entire energies of vast numbers of the human race are stimulated. Mountain masses have been arranged in groups, to which geographers have assigned various names. Elaborate attempts have also been made to classify mountains with reference to geological systems; and numerous observations have been made by active and laborious travellers in pursuit of these views. Mountains may also be denominated the 66 walls of nations," as they divide the nations of the earth.

It may perhaps be possible and profitable to popularize one or two of the more striking amongst the later trophies of science in the department of physical geography. Selecting the subject of isothermal lines, which was first treated of by the eminent traveller Humboldt, it may be observed, that they are curves so drawn upon a geographical map as to connect together all the places on the surface of the globe which have the same mean annual temperature; and this mean is deduced from a comparison of observations made during a series of years. As the average amount of heat at the earth's surface depends on latitude, the mean temperature of places situated on the same elevation decreases gradually from the equator towards the poles. Yet this decrease

does not occur regularly, for in fact the isotherms, or lines of annual equal temperature, form various curvatures or sinuosities in the course of their direction; and they diverge more and more from the parallels of latitude as they recede from the equator.

It will thus be clear that a number of isothermal zones, or belts, may be constructed from the comparison of the several observations of mean annual temperature registered in different countries. As these registrations increase in extent and minuteness, we shall be able to cover the globe, as it were, with circles of equal temperature, and to form maps of climate, without regard to the other mathematical divisions by which ordinary maps are now set out, and without reference to astronomical regulations. If our object be to ascertain the limits of the isothermal zones, that is, the extent of two physical spaces on the globe within two lines of equal average temperature of the year, then we shall confine ourselves to the indications of the thermometer. Thus the probable line of greatest temperature of the atmosphere, that is, the atmospheric thermal equator, will not coincide with the equator of the earth, but intersect in the northern hemisphere 255°, and in the southern only 105° of the circumference so that the proportion between the two hemispheres is nearly, in decimals, 142-4.

We might proceed to designate and determine very nearly the names and limits of the hot or equatorial zone, the warm zone, the mild zone, and then of the cold, and the frigid or polar zones. Thus a fair climatic map of the globe might be

D

exhibited. In an isothermal chart, the degrees of mean annual temperature (Fahrenheit) are marked at the sides of the ordinary degrees of latitude, and numbered by the scale of temperature, which scale alone we regard when speaking on this subject. One example may suffice, and it will include the portions of most interest in our design.

Between the isothermal curves 60° and 70° (these figures referring exclusively to degrees of temperature) lie a central portion of South America, the Cape of Good Hope, and nearly the lower half of Australia. The warm zone will comprehend the limits between the isothermal curves of 77° and 59°; and the mean annual temperature of this district will be 68°. This comprehends on one side the whole of the isles of Polynesia, a large portion of South America, the whole of South Africa, and, on the other side, almost the whole of the island of Australia. The isothermal line of 54.5° has been traced nearly round the earth.

It will always be remembered that we speak of the mean annual temperature in adverting to the isothermal curvature which passes over any locality. Much very interesting information might be accumulated on the subdivisions of the same mean annual temperature into their local and temporal variations in the different seasons. For example, Hungary and Dublin are both situated on the same isothermal line, although the temperature of the month of August in Hungary is 69.8°, while at Dublin at most it is only 60.8°. A number of these instances would show in what various measures the same mean annual tempera

ture may be distributed amongst the different seasons, and what an important influence these exercise, in the course of a year, on vegetation, agriculture, the ripening of fruits, and the material welfare of man.

This kind of observation has been extended to the ground itself, and lines have been laid down which pass through places where the mean temperature of the earthy material of the upper strata of the globe is always the same: these are called isogeothermal lines, the literal import of which term is, lines of equal-earth-warmth. Now, it is a connecting fact in this science, that the isogeothermal lines, of which we are speaking, are always parallel to the isothermal lines of which we have already spoken; consequently, the same general formula will serve to determine both, since the difference is a constant quantity, obtained by observation, and depending upon the distance of the place from the neutral isothermal line.

Places may evidently have the same mean annual temperature, and yet differ materially in climate. In one the winters may be mild, and the summers cool; whereas another may experience the extremes of heat and cold. Lines passing through places having the same mean summer or winter temperature, are neither parallel to the isothermal or geothernal lines, nor to one another, and they differ still more from the parallels of latitude.

The latitude of two places in Europe which have

These names, though now uncommon, may become usual in books, and are therefore introduced and explained here.