over the globe, as deducible from the thermometrical variations of the air, we are far from considering these laws as the only ones necessary to resolve all the problems of climate. Most of the phenomena of nature present two distinct parts, one which may be subjected to exact calculation, and another which cannot be reached but through the medium of induction and analogy. Having considered the division of heat between winter and summer on the same isothermal line, we shall now point out the numerical ratios between the mean temperature of spring and winter, and between that of the whole year and the warmest month. From the parallel of Rome to that of Stockholm, and consequently between the isothermal lines of 60°.8 and 41°, the difference of the months of April and May is everywhere 10°.8 or 12°.6, and all the successive months are those which present' the most rapid increase of temperature. But, as in northern countries, in Sweden, for example, the month of April is only 37.4, the 10°.8 or 12.6 which the month of May adds*, necessarily produces there a much greater effect on the developement 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 analogous 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 7.2. 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 sys 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 1°.44, while the differences rise in autumn from 3o.6 to 5o.4, and in spring from 5o.4 to 7°.2.— H. tem 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, wherever 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 between 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, (15°.7, 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 shews itself anew, and almost as great, and as much prolonged, as near the Arctic concave summit. The differences of the four months are 10°.4; 13.9; and 10°.8. In the curve of annual temperature, the spring and autumn mark the transitions from the minimum and the maximum. The increments are naturally slower near the summits than in the intermediate part of the curve. Here they are greater, and of longer continuance, in proportion to the difference of the extreme ordinates. The autumnal decrease of temperature is less rapid than the vernal increase, because the sur 2 uniform the laws are which I have just established. which it has acquired. The following Table will prevails in autumn, the earth loses slowly, by radiation, the heat atmosphere, and because, in spite of the serenity of the air which face of the earth acquires the maximum of heat slower than the shew how Differences of Tem Mean March. April. May. June. perature of the Four Temp. of Months. the Year. 7°.2 64°.8 5.4 11.2 58.1 39 0 43 7 57.4 61.2 70.9 13.7 3.6 9.7 53.8 Philadelphia, 39 56 44.1 53.6 62.1 72.3 9.5 8.5 10.3 53.6 New York, 40 40 38.7 49.1 65.8 80.2 10.4 16.7 14.4 53.8 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, 41°.9, the Peach-tree (Amygdalus Persica) flowers. 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 Petersburgh, 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 5o.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 vegetation, 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. Cotte, Meteorologie, p. 448.—Wahlenberg, Flor. Lap. Pl. 51. + Playfair, Edin. Trans, vol. v. p. 202.-Wahlenberg in Gilbert's Annalen, tom. xli. p. 282. |