a greater or less distance according to local circumstances, and producing, as we have already seen, so chilling an effect on the climate of Labrador; and probably yet more influencing that of the island of Georgia, in the southern hemisphere. But the water, when arrived in the equatorial regions, (like the aërial currents already described,) has not a rotatory motion of equal velocity with that of this portion of the globe; and, consequently, has an apparent motion from east to west. Water, also, like air, has less density at a high temperature than when it is cold; and thus the greater warmth of the ocean in intertropical regions will cause it to flow in an upper current towards the poles. A mutual interchange is, therefore, continually taking place between the waters of the warm and cold regions of the globe.

The equatorial, or easterly current, is very apparent, both in the Atlantic and the Pacific, between the parallels of 30° on each side of the equator; having a mean velocity of nine or ten miles per day in the open sea. The waters which form this current, being generally about three or four degrees cooler than those of the waters under the line, tend to moderate the heat within the tropics.

The most powerful and influential of known currents, however, is that called the Gulf Stream, which has its source in the Gulf of Mexico, where the water, like that of the Mediterranean, and other inland seas in warm latitudes, has a higher temperature than the wide ocean in similar parallels. The temperature of the water in the Mexican Gulf, in summer, is 86°, this being at least seven degrees higher than that of the Atlantic in the same latitude. From this great reservoir of warm water, a constant current pours through the Straits of Bahama, taking a north-easterly course, at the rate of three or four miles an hour. As it proceeds northwards, it flows with diminished velocity, until it is lost on the banks of Newfoundland, where it encounters the great current from the Polar regions. Its course may be traced, not only by its effect in retarding, or

speeding, the progress of ships, but more particularly, by the higher temperature it retains; for, in north latitude 42', its waters have still a temperature of 71o, whilst that of the adjacent sea, beyond its influence, is 8° lower, or 63°. It reaches the Azores in seventy-eight days, after flowing nearly 3,000 geographical miles*. From thence, extending its course 1,000 miles further, it arrives at the Bay of Biscay, still retaining an excess of 5° above the mean temperature of the sea. And as it has been known to reach this spot in the months of November and January, it probably tends greatly to moderate the cold of winter in Western Europe; along the whole shores of which it is supposed to extend: an inference drawn, not merely from the comparatively high temperature of the British Isles, and of that portion of the continent, but also from the fact, that various seeds and other tropical productions, are occasionally deposited on the shores, even as far north as Norway.

A large tract occurs in the centre of the North Atlantic Ocean, between the parallels of 33° and 35° north latitude, not much less than 2,000 miles in length from east to west, and about 350 in width, which has been called by Major Rennell, "the recipient of the gulf water." A considerable portion of this area is covered by the sea-weed called sargasso, which the current floats in abundance from the Gulf of Mexico. This mass of water is nearly stagnant, and is warmer by 7° or 10° than the waters of the Atlantic in that parallel; its heat being maintained by constant supplies of warm water from the south; and there is reason to conclude that the general climate of some parts of Europe and America, may be materially affected by this vast surface of warm

water.

It thus appears, that the diversities of climate are chiefly dependent on latitude; on the distribution of land and water; and on the elevation of land above the sea: as also on the nature of the soil; the prevalence of particular winds; on currents of the ocean; and some other local circumstances.

* About 3,450 British statute miles.

The causes which affect climate being thus various, it will be evident that great differences of temperature will prevail in different regions, even though situated in similar parallels; and it can only be from actual observations, that the distribution of heat over the earth's surface can be ascertained. This has, in great measure, been effected by De Humboldt; who, from accumulated observations, made by himself and others, has deduced the lines of equal temperature over a considerable portion of the globe. These lines, which are termed isothermal lines*, are, excepting perhaps within twenty-two degrees of the equator, neither parallel with that line, nor with each other; being, generally speaking, higher in the western than the eastern regions, both in the Old and New World; and descending lower in America than in Europe; this difference increasing as we recede from the equator. Thus, the isothermal line of 59° Fahrenheit, passes in Europe between Rome and Florence, in latitude 43°; but near Raleigh, in North Carolina, it descends to lat. 36°. The line of mean equal temperature of 50°, passes through the Netherlands in lat. 51°; whilst at Boston, in the United States, it descends to lat. 42°. The line of the freezing point passes through Uloa, in Lapland, between 66° and 68° north lat.; and at Table Bay, on the coast of Labrador, it descends to lat. 54°, a difference of above 12° of latitude occurring in the latter instance.

When we speak of the mean annual temperature of any place, or places, it is, however, by no means necessarily implied, that their climates should correspond. The mean temperature of any station is determined by ascertaining the average temperature of each day in the year, and taking the mean of the whole. And thus, if we were desirous of knowing the mean temperature of the coldest day of winter, and the hottest day of summer, at any two given stations, and we find those of the one station to have been respectively 400 and 60°, and those of the other to have been 20° and 80'; if we add each of these together, and divide

* From isos, equal, and thermé, heat.

them by two, we shall perceive that the mean temperature at both stations will have been 50°; but it will be evident, that though this may be similar, the climate of the two stations will differ.

Such we shall find to be the case with the annual temperature of various places through which these isothermal lines may pass. In some, the winters may be mild and the summers only moderately warm, whilst in others the extremes of heat and cold may be experienced. Thus, the mean annual temperature of England is 50°, the mean summer temperature being about 63o, and the mean winter about 37°, making a difference of twenty-six degrees; at Pekin the mean annual temperature is 51°, the summer temperature being 79° and the winter 23°, making a difference of not less than fifty-eight degrees. The mean annual temperature in these cases is not widely different, but the climate of the two places is wholly dissimilar, the one possessing an insular, and the other an excessive climate.

The difference between the mean summer and winter temperature is usually much less between the tropics than in temperate zones. Thus, at Cumana, which is situated in 10° north latitude, there is only a difference of four degrees between the temperature of the warmest and coldest months.

The greater part of the above-mentioned variations, are accounted for by the causes of diversity of climate already enumerated. Some of the phenomena of temperature and climate appear, however, as far as our present knowledge extends, incapable of explanation by these ordinary causes of variation. Such is the supposed occurrence of two regions of maximum or greatest cold on both sides of the equator. The position of those in the southern hemisphere has not been determined. In the northern hemisphere the regions, or as they are termed meridians of greatest cold, are both situated in about the eightieth parallel of latitude, and (widely speaking) about 100° east and west longitude. The mean annual temperature at the North Pole, is supposed to be about 4° or 5° Fahrenheit; the temperature at these

stations is considerably lower; that of the eastern or Asiatic meridian being nearly 1° Fahrenheit; and that of the western or American meridian of greatest cold, about 34° below zero. It is not impossible that future researches may show, that these meridians of greatest cold, are attributable to the exclusion of those regions, from causes tending to moderate their climate.

The innumerable diversities on the surface of the globe, to which the various combinations of climate, soil, and temperature give rise, adapt the earth in a remarkable manner for the organized beings with which it is covered. Had a uniform climate been communicated to the whole earth, or even had there been no undulations on its surface, and no alternations of land and water, its productions would, in all probability, have been comparatively limited in variety and number; as at present constituted, however, we find that the bountiful Creator has not only given us abundance, but also almost endless variety, affording us unbounded scope for research, and for admiration of the supreme wisdom which is displayed in every part of the natural world; and thus leading us to feel and to acknowledge that