May adds, necessarily produces there a much greater effect on the development of vegetation than in the south of Europe, where the mean temperature of April is from 53.6 to 55.4. It is from an ana. logous cause that, in passing from the shade to the sun, either in our climates in winter, or between the tropics on the back of the Cordilleras, we are more affected by the difference of temperature than in summer and in the plains; though in both cases the thermometrical difference is the same,-for example, from 5.4 to 72. Near the polar circle, the increase of the vernal heat is not only more sensible, but it extends equally to the month of June. At Drontheim, the temperatures of April and May, like those of May and June, differ not 10.8 or 12.6, but 14.4 or 16*2. In distinguishing upon the same isothermal line the places which approach its concave or convex summits, in the same system of climates in the northern and southern regions, we shall find,— 1st. That the increase of the vernal temperature is great, (from 14.4 or 16'2, in the space of a month,) and equally prolonged, where. ever the division of the annual heat between the seasons is very unequal, as in the north of Europe, and in the temperate part of the United States. 2dly. That the vernal increase is great, (at least above 9° or 10'8,) but little prolonged, in the temperate part of Europe. 3dly. That the increase of the vernal temperature is small, (scarcely 7.2,) and equally prolonged, wherever there is an insular climate. 4thly. That in every system of climates, in the zones contained be. tween the same meridians, the vernal increase is smaller, and less equally prolonged, in low than in high latitudes. The isothermal zone from 53.6 to 55'4 may serve as an example for confirming these different modifications of spring. In Eastern Asia, near the concave summit, the differences of temperature between the four months of March, April, May, and June, are very great, and very equal, (157, 13.3, and 13.9.) In advancing westward towards Europe, the isothermal line rises again; and, in the interior of the country, near the convex summit, the increase is still greater, but little prolonged: that is to say, that, of the four months which succeed one another, there are only two whose difference rises to 13°; they are 9'4, 13′3, 4′1. Farther west, on the coasts, the differences become small and equal,—viz. 3·6, 6·5, 5.6. In crossing the Atlantic, we approach the western concave summit of the isothermal line of 53.6. The increase of vernal temperature shows itself anew, and almost as great and as much prolonged, as near the Arctic concave summit. The differences of the four months are 104, 13′9, and 10'8. In the curve of annual temperature, the spring and autumn mark the transi tions from the minimum and the maximum. The increments are na turally slower near the summits than in the intermediate part of the curve. Here they are greater, and of longer continuance, in propor * In calculating for Europe, from 46° to 48° of lat, for ten years, the mean temperatures of every ten days, we find that the decades which succeed one another differ near the summits of the annual curve only 141, while the differences rise in autumn from 3'6 to 574, and in spring from 5'4 to 7·2. tion to the difference of the extreme ordinates. The autumnal decrease of temperature is less rapid than the vernal increase, because the sur. face of the earth acquires the maximum of heat slower than the atmosphere; and because, in spite of the serenity of the air which prevails in autumn, the earth loses slowly, by radiation, the heat which it has acquired. The following Table will show how uniform the laws are which I have just established: 1. Continental Climate: Rome.... 57. O 44.1 53.6 62.1 72.3 9.5 8.5 10.3 53.6 80.2 10.4 167 14.4 53.8 70.2 11.0 11.3 13.3 50.4 63.9 16.6 15.1 41.2 41.7 20.7 9.5 8.1 26.4 41.53 50:4 55.4 66.9 72.3 5.0 11.5 5.4 Ulco 65° 0 Enontekics 68.30 14:0 26.2 41.0 55.0 12.2 14.8 14.0 11.5 26.6 36.5 49.5 53.1 15.1 9.9 13.0 27.0 41'4 47.3 50.5 57.2 5.8 3.2 6.7 71 O 25.0 30.0 34.0 40.1 47.8 5.2 4.0 6.1 32.0 39.54 41.4 70.3 57.0 84.2 15:7 13.3 13.9 54.9 In all places whose mean temperature is below 62-6, the revival of nature takes place in spring, in that month whose mean temperature reaches 42.8 or 46.4. When a month rises to, 419, the peach-tree (amygdalus persica,) flowers, 518, the birch-tree* (betula alba,) pushes out its leaves. At Rome, it is the month of March, at Paris the beginning of May, and at Upsal the beginning of June, that reaches the mean temperature of 51.8. Near the hospice of St. Gothard, the birch cannot vegetate, as the warmest month of the year there scarcely reaches 46.5. Barley, in order to be cultivated advantageously, requires, + during ninety days, a mean temperature of from 47.3 to 48.2. By adding the mean temperatures of the months above 51.8,—that is, the temperatures of those in which trees vegetate that lose their foliage,— we shall have a sufficiently exact mean of the strength and continuance of vegetation. As we advance towards the north, vegetable life is confined to a shorter interval. In the south of France there are 270 days of the year in which the mean temperature exceeds 51.8; that is to say, the temperature which the birch requires to put forth its first leaves. At St. Petersburg, the number of these days is only 120. These two cycles of vegetation, so unequal, have a mean temperature which does not differ more than 5·4; and even this want of heat is compensated by the effects of the direct light, which acts on the parenchyma of plants in proportion to the length of the days. If we compare, in the following Table, Eastern Asia, Europe, and America, we shall discover, by the increase of heat during the cycle of vegeta tion, the points where the isothermal lines have their concave summits. The exact knowledge of these cycles will throw more light on the problem of agricultural geography, than the examination of the single temperatures of summer. In the system of European climates, from Rome to Upsal, between the isothermal lines of 59° and 41°, the warmest month adds from 16.2 to 18° to the mean temperature of the year. Farther north, and also in Eastern Asia and in America, where the isothermal lines bend towards the equator, the increments are still more considerable. * COTTE, Meteorologie, p. 448.—WAHLENBERG, Flor. Lap. pl. 51. † PLAYFAIR, Edin. Trans. vol. v. p. 202.-WAHLENBERG in GILBERT's Aunalen, tom. xli. p. 282. As two hours of the day indicate the temperature of the whole day, there must also be two days of the year, or two decades, whose mean temperature is equal to that of the whole year. From the mean of ten observations, this temperature of the year is found at Buda in Hungary, from the 15th to the 20th of April, and from the 18th to the 23d of October. The ordinates of the other decades may be regarded as functions of the mean ordinates. In considering the temperatures of entire months, we find that, to the isothermal line of 35.6, the temperature of the month of October coincides (generally within a degree,) with that of the year. The following Table proves that it is not the month of April, as Kirwan affirms, (Estimate, &c. p. 166,) that approaches nearest to the annual temperature. As travellers are seldom able to make observations for giving immediately the temperature of the whole year, it is useful to know the constant ratios which exist in each system of climates, between the vernal and autumnal temperatures, and the annual temperature. The quantity of heat which any point of the globe receives, is much more equal during a long series of years than we would be led to believe from the testimony of our sensations, and the variable product of our harvests. In a given place, the number of days, during which the N.E. or S.W. winds blow, preserve a very constant ratio, because the direction and the force of these winds, which bring warmer or colder air, depend upon general causes,-on the declination of the sun,-on the configuration of the coast,-and on the lie of the neighbouring continent. It is less frequently a diminution in the mean temperature than an extraordinary change in the division of the heat between the different months, which occasions bad harvests. By examining, between the parallels of 47° and 49°, a series of good meteorological observations, made during ten or twelve years, it appears that the annual temperatures vary only from 1.8 to 2.7: those of winter, from 3.6 to 5.4; those of the months of winter, from 9° to 10.8. At Geneva, the mean temperatures of twenty years were as follow: |