And o’er unhabitable downs
Place elephants, for want of towns.”
The Society for Promoting the Discovery of the Interior Parts of Africa has been fortunate in collecting much geographical information, in so early a stage of the African researches; and there is little doubt but that in a few years all the great features of this Continent (within the reach of their enquiries) may be known and described. But to accomplish this, it will be necessary that intelligent Europeans should trace some of the principal routes; as well to apportion the distances, as to establish some kind of criterion for the parole information derived from the natives. As yet, in the wide extent of near thirty degrees on a meridian, between Benin and Tripoli, not one celestial observation has been taken, to determine the latitude.
But as far as materials composed by a scale founded on estimated distances (that is, days journeys of caravans) may go towards the establishment of geographical positions, the itineraries made use of for constructing the new matter in the accompanying map, are less discordant than might be expected in so wide an expanse, and on the foundation of rules so vague as those which necessity has compelled me to apply. On this, however, the Reader must exercise his own judgement, after the following exposition of the data, and the rules by which I have determined the scale.
It will easily be conceived by the Reader, that all roads, except such as are made through a country, in which the public works are in the highest state of improvement, and where also the face of it is perfectly level (the curvature of the earth excepted) must have some degree of inflexion, both horizontally and vertically; and either of these will occasion a Traveller to trace a line of greater length by the road, than can be measured on a straight line, drawn from the point of commencement, to that of the termination of his journey. The quantity of the difference must vary with the nature of the country; but in ordinary cases, still more with the extent of the line of distance: for a different ratio between the road distance and horizontal distance must obtain, as the line of distance is increased. Let it be admitted, that in stages of ten or twelve miles, the winding of the road occasions a loss of only one-tenth part, which may be termed the simple winding: yet as the different stages in an extent of 100 or 150 miles, do not lie in a straight line, drawn through the whole extent, but often very far to the right and left of it, a compound winding arises: and I have found by long experience, that one mile in eight must be deducted, to reduce the road measure, on such a length, to horizontal measure. When a line of distance is extended to 500 miles and upwards, the rule becomes much more vague than when applied to moderate distances; because it often happens (and more particularly in unimproved countries) that obstacles present themselves, and give an entire new direction to the course of the road; although the two parts of it, considered separately, may have only an ordinary degree of crookedness. However, as some of the lines of distance applied to the present subject, are from thirty to forty days journey, it becomes necessary that some general rules should be adopted. It happens that examples are furnished, in two cases, on very long journies, where the real distances between the terminating points of the routes are nearly known: such is that of fifty-three days journey, between the Capital of Fezzan and Cairo; and the mean horizontal distance for each day, is fourteen and a half geographic miles, or those of sixty to a degree. I confess I should have expected much less. The other example is between Arguin and Gallam: there forty days produce thirteen miles for each day; and this is conformable to my expectations. In the examples of small distances, such as six days journey, sixteen miles per day is the result, and is consistent. For a caravan journey, taken at twenty-two British miles of road distance, will produce, when the allowance for winding is deducted, and the remainder reduced to geographic miles, about sixteen and a half such miles for a single day.
The following are the proportions which I have established, for the application of a scale, to the different degrees of distance.
For one day, sixteen miles and a half; for seventeen to twenty-five days, fifteen miles; for forty to fifty days, thirteen miles. These numbers are particularly selected, because they occurred in the course of the Work. The Reader will be pleased to observe, that the miles spoken of in the construction, are always those of sixty to a degree of a great circle. However tedious this investigation may appear to the generality of readers, it is absolutely necessary; as it is the hinge upon which the whole turns: and a neglect of attention to this particular subject, would warrant the Reader’s taking the whole for granted, without further examination.
Mr. Beaufoy having given, from the materials in his possession, so full an account of each road and country, nothing remains for me to do, but simply to describe, from the same materials, the mode of fixing the principal positions, in the Map. As the object of it is to exhibit the new matter only, care has been taken to exclude all that has already appeared, except what was absolutely necessary towards explaining the other: and as the borrowed particulars are distinguished from the rest, the Reader cannot be mistaken. The outline of the great body of Africa, together with the courses of the Nile, Gambia, Senegal, and Wad-drah, are copied from Mr. D’Anville.
Fezzan (or rather its capital, Mourzouk) is given in the Itineraries at the distance of seventeen days and half from Mesurata. These, taken at fifteen miles per day, produce two hundred and sixty-two miles. The bearing is said to be South from Mourzouk; and this latter is placed according to D’Anville. Mourzouk, then, falls in latitude 27° 20′.
Agadez, the next principal station, is, at a medium forty-one days from Mourzouk, on a South West course, or thereabouts: and these, at thirteen miles per day, produce four hundred and fifty-five miles; and place Agadez in latitude 20° 20′; and nearly in the meridian of Tripoli. Agadez is the Agadost of Edrisi.