BOOK thography, not absolutely fixed, (which would perhaps be useless,) but at least easy to follow and comprehend.* VI. Signs of physical geography. The physical part of a map requires attention to be paid to certain other circumstances. It is desirable to know if a country is covered with plains, or is rough with mountains, naked or wooded, dry or marshy. Designers have introduced signs, either conventional, or of the nature of pictured representations, to express, on trigonometric surveys, and topographical plans, these different circumstances, which, joined with the climate and the laws of meteorological phenomena, determine the physical geography of each country. It is sufficient to cast one's eyes on plans of this kind, to discover the signs employed in them; they are all conformaable to the rules of bird's-eye perspective; thus the parts more or less strongly shaded, represent slopes more or less steep, on which the light is lost the more they approach the vertical position. Geographical maps are less calculated to admit of this improvement, especially with regard to mountains; for the scale of those maps is necessarily too small to admit of expressing on them, in just proportions, the innumerable inequalities of ground, from the highest chains of mountains, to hills of the lowest order. Formerly, mountation of tains used to be represented by slight elevations in profile, mountains, which supposed the eye of the spectator to be in the plane Represen of the map. At present, they attempt to represent in a bird's-eye view the chains and groupes of mountains, and even the peaks or insulated points which repose in general on elevations more or less considerable, but the whole extent of which presents contours that determine the form of valleys.† The new method would undoubtedly be preferable, if one could preserve a just proportion between the different elevations, and if we possessed all the necessary information for determining, point by point, the level of the ground. But, as long as these elements are Comp. Langlès, Préface du Voyage de Norden; Volney, sur l'Alphabet Russe, &c. + Memorial Topographique et Militaire, cab. v. wanting, the new method will be as arbitrary and illusory BOOK as the old one appears unnatural and unsatisfactory. The partisans of the plan of representing mountains by a bird's-eye-view, shew us the maps of D'Anville, and exclaim, "how vague and insignificant are these mountains, marked with insulated points! All that we see is, that the country they occupy is mountainous; one might as well write down there are mountains here; nothing indicates the course of chains, their various sinkings, and their connexions, either with each other, or with the islands which form the summits of the chains of submarine mountains, or which traverse the basin of the sea." But, in the first place, there are many other maps besides those of D'Anville, in which the mountains, though expressed in profile, please the eye and satisfy the mind. And we ask, in our turn, if geography has really gained by the admission of all these pretended chains, either terrestrial or submarine, which M. Buache, sen. has created, by supposing arbitrarily that all the basins of rivers are separated by considerable heights. VI. The pretensions of topography have been pushed still Method proposed farther. A geographical engineer, M. Dupain-Triel, has for exhibi published a method, by which a geographical map may be ing levels. made to indicate the elevation of each point of the ground. He observes, that if we join by a line drawn on a marine map, all the points at which equal soundings are marked, this line would give the contour of a section made at the bottom of the sea by an horizontal plane at such a depth below the surface of the fluid, as is expressed by the number of measures contained in the sounding. From this observation, just in itself, he thinks he may deduce a principle for representing geometrically the configuration of the surface of a country. This consists in tracing on the Dupain-Triel, Carte intitulée, Méthode nouvelle pour exprimer sur les cartes les hauteurs, etc. avec un mémoire de M. Du Caila. Paris, 1784. Id. Carte de la France, où l'on a essaye, etc. An. vii. VI. BOOK map to be constructed, lines which pass through points placed at the same level, or at the same height, above the surface of the sea; lines which would become successively the boundaries of its waters, if it rose, by any cause whatever, to the level they occupy; as the lines which join equal soundings would become in their turn the shores of the sea, if sunk by the number of measures marked on those soundings. The heights of these lines or horizontal sections of the ground could be graduated, according to the scale of the map and the steepness of the slopes. On a specimen of maps of France executed according to this plan, M. Dupain-Triel traced in parts almost flat, and near the sea, a line passing through the points elevated 10 toises; then another passing through those elevated 20, and so on from 10 to 10 toises. We see these lines, at first pretty distant from each other, become closer in proportion as the country rises more rapidly. Round the insulated mountains, the line of level, which is marked only for differences of 50 toises, and even 100, come closer the more the slopes are steep. Table-lands are indicated by lines of level, which turn round them. Finally, if we conceive lines which cut the lines of level at right angles, we shall have the lines of the greatest declivity, or those which follow in their descent the waters spread on the sides of mountains. Value of this me thod. Though this method of M. Dupain-Triel is not new, having been already proposed by Ph. Buache and others,* it undoubtedly merits some attention. It is evident that it furnishes descriptive geometry with means for resolving problems on the succession of table-lands, the intersection of slopes, and the intervention of basons; problems interesting in the construction of roads and canals. It might offer the means of collecting and putting within the reach of every body a multitude of levellings and observations, made by military and civil engineers, on the heights of mountains, the results of which are confined to the portfolios of * Mem. de l'Acad. des Scien. 1752, p. 399; 1753, p. 586; et 1756, p. VI. government: finally, the advantage to be derived from it BOOK would excite travellers, and philosophers residing in all great towns, to multiply barometrical observations in order to determine the respective heights of the places where they are made. But, till the elements of such a map are more numerous and more authentic than those which we possess, its execution would only serve to give an air of reality to ystematic ideas that are very uncertain. At all events, the confusion which would result from this multiplicity of lines, would prevent any object of political or historical geography from being clearly designed on these maps. We must therefore consign all inventions of this kind to maps, specially consecrated to physical geography, in the same way as the details of hydrography are reserved for nautical charts. General BOOK VII. Continuation of the Theory of Geography. First sketches of HAVING Considered the earth in regard to its dimensions, we must now study its PHYSICAL characters. This part of our work, which is perhaps the most interesting of all, will necessarily be the most imperfect; because a good system of physical geography, can only be the gradual work of many successive ages. This science, before it can make advances to maturity, views of requires a continual series of observations, both repeated physical geography, and varied, made in every part of the world, and so combined, as to leave no interval unoccupied. On the other hand, it is not with natural geography as with mineralogy, with chemistry, or with botany. Ingenious arrangement, and exact and methodical classification, are not very applicable to it, and for some time would only retard its progress, by loading it with a display of illusory notions. Mountains, valleys, waters, climates, and tracts of country, present themselves to the eye under very complicated and irregular appearances, which it is much easier to describe than to bring within exact definitions. The grandeur and majesty of nature, defy the subtilty of our combinations, and the littleness of our rules. The spirit of physical geography, unquestionably rejects vague and incorrect language; but at the same time it cannot obviously be susceptible of the precision of terms, which belongs to mathematics or chemistry. What a striking dif |