... 63 50 33 3 68 30 27 0 Edinburgh Difference V. Parallels of Labrador, the South of Enontekies .... 11 0 17 1 This table* indicates the difference of climates, expressed by that of the mean temperature, and by the number of degrees in latitude which it is necessary to go northward in Europe, in order to find the same quantity of annual heat as in America. As a place could not be found in the Old World whose mean temperature was 48°, the same as that of Williamsburg, I have supplied it with an interpolation be-tween the latitudes of two points whose mean temperatures are 56.5 and 59.4. By an analogous method, and by employing only good observations, I have found that 1. The isothermal line of 32° (0° centig.) passes between Uleo and Enontekies in Lapland, (lat. 66° to 68°; East long. from London 19° to 22°,) and Table Bay in Labrador, (lat. 54° 0'; W. long. 58°.) 2. The isothermal line of 41° (5° centig.) passes by near Stockholm, (lat. 60°, East long. 18°,) and the Bay of St. George in Newfoundland, (lat. 48°, and long. 599.) 3. The isothermal line of 50° (10° centig.) passes by Belgium, (lat. 51°, East long. 2o,) and near Boston, (lat. 42o 30', West long. 70° 59.) * See my Prolegomena de Distributione Geographica Plantarum, secundum Cœli· temperiem et altitudinem montium, p. 68, 6 4. The isothermal line of 59° (15° centig.) passes between Rome and Florence, (lat. 43° 0', East long. 11° 40',) and near Raleigh in North Carolina, (lat. 36° 0', and West long. 76° 30′.) The direction of these lines of equal heat gives, for the two systems of temperature which we know by precise observations,-viz. part of the middle and west of Europe, and that of the coast of America, the following differences: If we call the mean equatorial temperature 1, we shall have the half of this temperature in the Old World at 45°, and in the east of the New World at 39° of lat. In both continents, the zone in which the mean temperature decreases most rapidly is comprehended between the parallels of 40° and 45°. Observation here presents a result entirely conformable to theory, for the variation of the square of the cosine, which expresses the law of the temperature, is a maximum towards 45° of latitude. We have traced the direction of the isothermal lines from Europe to the Atlantic provinces of the New World. We have seen them ap-. proach one another from parallelism towards the south, and converge towards the north, particularly between the thermometric curves of 41° and 50°: we shall now endeavour to pursue them to the west. North America presents two chains of mountains, extending from N.E. to S.W. and from N.W. to S.E. forming almost equal angles with the meridian, and nearly parallel to the coasts which are opposite to Europe and Asia,-viz. the chain of the Alleghanys and the Rocky Mountains, which divide the waters of the Missouri and the Columbia. Between these chains stretch the vast basin of the Mississippi, the plains of Lousiana and of the Tenesse, and the states of Ohio, the centre of a new civilization. It is generally believed in America that the climate is more mild to the west of the Alleghany mountains, than under the same parallels in the Atlantic states. Mr. Jefferson has estimated the difference at 30 of latitude; and the gleditsia monosperma, the catalpa, and the aristolochia sypho, and other vegetable productions, are found so many degrees farther to the north, in the basin of the Ohio, than on the coast of the Atlantic.* Mr. Volney has * See my Essai sur la Geographie des Plantes, p. 154. 2 Q No. 271. endeavoured to explain these phenomena by the frequency of the south-west winds, which drive back the warm air of the Gulf of Mexico towards these regions. A series of good observations, made, for seven years, by Colonel Mansfield at Cincinnati, on the banks of the Ohio, and recently published by Mr. Drake, in an excellent trea tise on American Meteorology,* has removed the doubts which ob.. scured this point. The thermometrical means prove that the isothermal lines do not rise again in the regions of the west. The quantity of Heat which each point of the globe receives under the same parallels is nearly equal on the east and west of the Alleghany range, the winters being only a little milder to the west, and the summers a little warmer.t The migrations of vegetables towards the north are favoured, in the basin of the Mississippi, by the form and the direction of the valley which opens from the north to the south. In the Atlantic provinces, on the contrary, the valleys are transverse, and oppose great obstacles to the passage of plants from one valley to another. If the isothermal lines remain parallel, or nearly so, to the equator, from the Atlantic shores of the New World to the east of the Mississippi and the Missouri, it cannot be doubted that they rise again beyond the Rocky Mountains, on the opposite coast of Agia, between 35° and 550 of latitude. Through 122° 40′ of west long. the isothermal line of 50° Fahr. appears to pass almost as in the Atlantic part of the Old World, at 50° of lat. The western coasts of the two worlds resemble one another to a certain point. But these returns of the isothermal lines do not extend beyond 60°. The curve of 32° Fahr. is already found to the south of the Slave Lake, and it comes still farther south in approaching Lakes Superior and Ontario. In advancing from Europe towards the east, the isothermal lines again descend,§ the number of fixed points being few. We can only Natural and Statistical View or Picture of Cincinnati and the Miami Country. 1 vol. 8vo. Cincinnati. The following comparison of the mean temperatures has been deduced with great care. CINCINNATI. Lat. 36° 6', West long. 84° 24'. PHILADELPHIA, Mean, 53.7 Mean, 53.5 I have taken for Philadelphia the means between the observations of Coxe and Rush. I have also referred, for correction, to the observations made by Mr. Legaux at Spring-Mill, upon the Schuylkill, to the north of Philadelphia. As Cincinnati is 512 feet above the level of the sea, its mean temp. is 14 too low. On account of the influence of west and south-west winds. See Dalton's Meteor. Observ. p. 125. In comparing places from the west to the east, and nearly under the same parallel, we find, Mean WEST. Lat. Mean EAST. Lat. Temp. Vienna, 48.13 505 Amsterdam, 52.21 53:4 Warsaw, Naples, Copenhagen, 40.50 63.3 Pekin, 39.54 54.9 Upsal, employ those which are made in places whose known elevation allows us to reduce the mean temperatures to the level of the sea. The few good materials which we possess have enabled us to trace the curves of 32° and 55.4.. We know even the nodes of the latter curve round the whole globe. It passes to the N. of Bourdeaux, (lat. 45'46, W. long. 0.37,) near Pekin, (lat. 39-54, E. long. 116-27,) and Cape Foul. weather to the S. of the embouchure of the Colombia, (lat. 44-40, W. long. 104°.) Its nodes are distant at least 162° of longitude. We have here pointed out only the empirical laws, under which are ranged the general phenomena and the variations of the temperature which em. brace at once a vast extent of the globe. There are partial inflexions of the isothermal lines, which form, so to speak, particular systems modified by small local causes; such as the strange inflexion of the thermometric curves on the shores of the Mediterranean, between Marseilles, Genoa, Lucca, and Rome, and those which determine the difference between the climate of the western coast and the interior of France. These last depend much less on the quantity of heat received by a part of the globe during the whole year, than upon the unequal distribution of heat between winter and summer. It will one day be useful to have, upon particular charts, the partial inflexions of the isothermal lines, which are analogous to the lines of soundings or of equal declivity. The employment of graphical representations will throw much light upon phenomena which are deeply interesting to agriculturists. If, in place of geographical charts, we possessed only tables containing the co-ordinates of latitude, longitude, and altitude, a great number of curious facts relative to the configuration and the superficial inequalities of continents would have remained for ever unknown. We have already found that, towards the north, the isothermal lines are neither parallel to the equator nor to one another; and it is on account of the want of parallelism that we have, in order to simplify such complicated phenomena, traced round the whole globe the curves of equal heat. The position of the line of 32° acts like the magnetic equator, whose inflexions in the South Sea modify the inclinations at great distances. We may even believe that, in the distribution of climates, the line of 32o determines the position of the curve of greatest heat, which is as it were the isothermal equator; and that, in America and Asia, through 78° of west and 102 of east longitude, the torrid zone commences more to the south of the tropic of Cancer, or that it there presents temperatures of less intensity. An attentive examina. tion of the phenomena proves that this is not the case. Whenever we approach the torrid zone below the parallel of 30°, the isothermal lines become more and more parallel to one another and to the earth's The elevation of Pekin is inconsiderable; that of Moscow is 984 feet. The absolute temperature of Madrid, to the west of Naples, is 59°; but the city is elevated 1978 feet above the level of the sea. Mean Genoa, 44.25 60.6 equator. The great colds of Canada and Siberia do not extend their action to the equatorial plains. If we have long regarded the Old World as warmer between the tropics than the New World, it is, 1st, because, till 1760, travellers used thermometers of spirit of wine, coloured, and affected by light; 2d, because they observed it either under the reflection of a wall or too near the ground, and when the atmosphere was filled with sand; and 3d, because, in place of calculating the true mean, they used only the thermometric maximum and minimum. Good observations give, The mean temperature of the equator cannot be fixed beyond 811o. Kirwan values it at 84°; but only two places of the earth were known, viz. Chandernagor and Pondicherry, to which old travellers attributed annual temperatures above 8110. At Chandernagor, in latitude 21.6, the mean temperature, according to Cotte, is 919; but the Jesuit Boudier marked only the days when the thermometer was above 98.6 and below 57.2: and at Pondicherry, in latitude 11-55, the mean temperature, according to Cotte, is 85.3, and according to Kirwan, 88°; but M. de Cossigny observed with a spirit-of-wine thermometer. The distribution of heat over different parts of the year differs, not only according to the decrease of the mean annual temperatures, but also in the same isothermal line. It is this unequal division of the heat which characterizes the two systems of climate of Europe and Atlantic America. Under the torrid zone, a small number of months are warmer in the Old World than in the New. At Madras, for example, according to Dr. Roxburgh, the mean temperature of June is 894; at Abusheer, 93.2; but at Cumana I have found it only 84.6. With respect to the temperate zone, it has long been known that, from the parallel of the Canary Isles to the Polar Circle, the severity of the winter augments in a progression much more rapid than the summers diminish in heat. It is also known that the climate of the islands and the coasts differs from that of the interior of continents, the former being characterized by mild winters and less temperate summers. But it is the heat of summer particularly which affects the formation of the amylaceous and saccharine matter in fruits, and the choice of the plants that ought to be cultivated. As the principal object of this memoir is to fix, after good observations, the numerical relations between the unequal quantities of heat distributed over the globe, we shall now compare the mean temperatures of three months of winter and summer under different latitudes, and show how the inflections of the isothermal lines modify these relations. In following the curves of equal heat from west to east, from the basin of the Mississippi to the eastern coasts of Asia, through an extent of 4000 leagues, we are struck with the great regularity which appears in the variations of the winter temperature. |