months or seasons of the year,—the amount and character of its rains,-and the electrical states and changes of the atmosphere. The secondary conditions, many of which may depend intimately on some one or more of the foregoing, are, the character, &c., of the vegetation, the degree of shelter from the winds, the extent of the drainage,-the character of the water used for domestic purposes,-the degree to which the existing animal or insect life affects the human population,-and the degree to which extinct and decaying organisms affect the health of the people.

The abstract question of temperature is one of the most important of the circumstances that modify man's adaptation to the globe he lives on. This seems, however, to be a more important matter, in regard to the risk incurred by those born in one latitude who migrate to another latitude, than as to the health and chances of life of the natives of the different regions of the earth. For, although the expectation of life is unquestionably greater in England than it is in most European countries, and greater in Europe than in America, and in America than in Asia, and in Asia than in Africa; yet the natives of the different regions of the world attain an average duration of life, which goes far to justify a belief that full and due attention to all the known means of health and preventives to disease, might almost serve to equalise the chances of life throughout the world, to those acclimated by hereditary circumstances, and usage from the infancy. But for a

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man whose organs and tissues have been gradually fashioned and accommodated to one set of physical circumstances, by the conditions of his own existence, and those of the existence of successive preceding generations,—as for example to the maintenance of life at a mean temperature of 50°, to migrate to a country of which the mean temperature is 80°, must militate much against the probable expectation of life, and as largely against the chances of health. Under such circumstances, the disturbing influences must be constant in their operation, and of importance in their character,-the power of adaptation, variable and generally insufficient,-and the risk of disturbance from the extra duty demanded of different organs must be considerable; and, accordingly, there must arise a liability to suffer from morbid conditions, comparatively little known to a native population acclimated to the temperature,-as well as a peculiar susceptibility to suffer from the indigenous ailments, common to the population of the country, and to suffer from them severely. The balance between the action of the different organs, by which the general expenditure of the system is regulated and secured, must be maintained, or health must suffer, and disease ensue. The skin, lungs, kidneys, liver, &c., are so many great emunctories for carrying away the effete and redundant matters from the economy; and these organs act relatively more or less, according to the temperature of the atmosphere. In cold air, the system must consume more carbon, and require a larger supply of oxygen; and the

lungs and the kidneys have more duty to perform in the same proportion ;-in warm weather, the skin, the liver, and the mucous membrane, become the more active organs, and the greater means of carrying off the water; and by so much are these organs severally more liable to derangement of their functions. As, in our own latitude, the greatest risk of disease is incurred when the temperature changes from heat to cold, or from cold to heat, and pulmonary or gastric affections become more or less general accordingly, so is health by so much damaged, the chances of life by so much lessened, and disease or its forerunners by so much increased, as man migrates to a climate that differs much in its average temperature from that of his birth-place.

The elevation of a district above the level of the sea, diminishes, in a definite ratio, its mean temperature, owing to the greater rarity of the air, and its correspondingly less capacity for caloric; to which causes must be added, in many cases, the greater exposure to the influence of winds, and a greater liability to electrical influences. It is a sufficiently near estimate for practical purposes, that the mean temperature of a district is lower by one degree of Fahrenheit's scale for every hundred yards of elevation; and, supposing this to be so, it serves to show, that, even at the equator, an elevation of some 15,000 or 16,000 feet, is within the region of perpetual snow, -and that the snow line is in these higher latitudes, at less than the fourth part of this elevation. Other influences, however, besides that of latitude, affect the

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elevation at which the snow-line commences: such as the exposure of the mountain side to the south or to the north, which may make a difference of several hundred feet in the elevation of the snow-line, when all other circumstances are apparently equal. Making all due allowance for such modifying circumstances, however, all the different climates of the world, as regards their respective mean temperatures, may be found within a couple of day's journey, gradually ascending from the burning plains of the tropics, to the everlasting snow of the mountain summits. In some instances, the climate of whole districts, or even whole countries, is thus modified by their elevation; and their mean temperature is found to be so much lower in proportion to their altitude. In noticing the principal climates and countries of the world, we find this to be singularly illustrated in the instances of the eastern and western sides of South America,-in the mountain lands of the interior of Jamaica, compared with the coast,-in the comparatively low mean temperature of the republic of Mexico, with reference to its latitude,in the mountain ranges of Asia, where the Himalayas and Neilgherries afford a European climate to the invalided and exhausted system of the AngloIndian. And not only is there the marked difference in the mean temperature, corresponding to the degree of elevation, but the equally marked difference in the vegetable productions and vegetable capabilities of the district; and the Anglo-Indian surrounds his mountain retreat with the fruit-trees,

and other garden-produce, of his far away native land. If an untravelled man might be allowed to envy the travelled any one of his experiences more than another, it might well be that enjoyed during an ascent from the plains of the tropics, with their burning temperature, and lurid atmosphere, and enervating land-winds, where grow the spice-plants and sugar-cane,-finding himself, as he ascends, leaving these, and the indigo-plantations, and the coffee-lands, and the rice-fields, the palms and giant ferns, gradually behind him, and, in the course of a few miles, passing through the districts of the cotton-plant, the maize, and the orange, to those of the grasses and cerealia, the olive and the vine,— thence to those of the oak, and other forest trees,in their turn, to lose all traces of these, the hardy pines only living in the colder and colder regions to which he ascends,-soon to leave even them behind, and the hardiest forms of vegetable life, and arrive at the regions of perpetual snow. These frozen districts exercise, in all latitudes, but more particularly within the tropics, an important duty, in tempering the air which passes over them, and moderating the heats of the districts that those winds subsequently pass over. The winds which have blown over large tracts of land in the interior of the great continents, exposed to the direct and vertical rays of the sun, become more and more heated by the untempered influence of the solar heat, the solar rays influencing the air directly in their passage through the atmosphere, and indirectly in their radiation from the earth again, so far