The history of geography shows us a great number of attempts to establish a uniformity of longitude, and when we look for the reasons which have caused those attempts (many of which were very happily conceived) to fail, we are struck with the fact that it appears due to two principal causes—one of a scientific and the other of a moral nature. The scientific cause was the incapacity of the ancients to determine exactly the relative positions of different points on the globe, especially if it was a question of an island far from a continent, and which consequently could not be connected with that continent by itinerary measurements. For example, the first meridian of Marinus of Tyre and Ptolemy, placed on the Fortunate Isles, in spite of its being so well chosen at the western extremity of the then known world, could not continue to be used on account of the uncertainty of the point of departure. That much to be regretted obstacle caused the method to be changed. It became necessary to fall back on the continent. But then, in place of a single common origin of longitude indicated by nature, the first meridians were fixed at capitals of countries, at remarkable places, at observatories. The second cause to which I just now alluded, the cause of a moral nature—national pride—has led to the multiplication of geographical starting-points where the nature of things would have required, on the contrary, their reduction to a single one.
In the seventeenth century, Cardinal Richelieu, in view of this confusion, desired to take up again the conception of Marinus of Tyre, and assembled at Paris French and foreign men of science, and the famous meridian of the Island of Ferro was the result of their discussions.
Here, gentlemen, we find a lesson which should not be lost sight of. This meridian of Ferro, which at first had the purely geographical and neutral character which could alone establish and maintain it as an international first meridian, was deprived of its original characteristic by the geographer Delisle, who, to simplify the figures, placed it at 20 degrees in round numbers west of Paris. This unfortunate simplification abandoned entirely the principle of impersonality. It was no longer then an independent meridian; it was the meridian of Paris disguised. The consequences were soon felt. The meridian of Ferro, which has subsequently been considered as a purely French meridian, aroused national susceptibilities, and thus lost the future which was certainly in store for it if it had remained as at first defined. This was a real misfortune for geography. Our maps, while being perfected, would have preserved a common unit of origin, which, on the contrary, has altered more and more.
If, as soon as astronomical methods had been far enough advanced to permit the establishment of relative positions with that moderate accuracy which is sufficient for ordinary geography, (and that could have been done at the end of the 17th century,) we had again taken up the just and geographical conception of Marinus of Tyre, the reform would have been accomplished two centuries sooner, and to-day we should have been in the full enjoyment of it. But the fault was committed of losing sight of the essential principles of the question, and the establishment of numerous observatories greatly contributed to this. Furnishing naturally very accurate relative positions, each one of these establishments was chosen by the nation to which it belonged as a point of departure for longitude, so that the intervention of astronomy in these questions of a geographical nature, an intervention which, if properly understood, should have been so useful, led us further away from the object to be attained.
In fact, gentlemen, the study of these questions tends to show that there is an essential distinction between meridians of a geographical or hydrographical nature and meridians of observatories. The meridians of observatories should be considered essentially national. Their function is to permit observatories to connect themselves one with another for the unification of the observations made at them. They serve also as bases for geodetic and topographical operations carried on around them. But their function is of a very special kind, and should be generally limited to the country to which they belong.
On the contrary, initial meridians for geography need not be fixed with quite such a high degree of accuracy as is required by astronomy; but, in compensation, their operation must be far reaching, and while it is useful to increase as much as possible the number of meridians of observatories, it is necessary to reduce as much as we can the starting points for longitudes in geography.
Further, it may be said that as the position of an observatory should be chosen with reference to astronomical considerations, so an initial meridian in geography should only be fixed for geographical reasons.
Gentlemen, have these two very different functions been always well understood, and has this necessary distinction been preserved? In no wise. As observatories, on account of the great accuracy of their operations, furnish admirable points of reference, each nation which was in a condition to do it connected with its principal observatory not only the geodetic or topographical work which was done at home—a very natural thing—but also general geographical or hydrographical work which was executed abroad, a practice which contained the germ of all the difficulties with which we are troubled to-day. Thus, as maps accumulated, the need of uniformity, especially in those that referred to general geography, was felt more and more.
This explains why this question of a single meridian as a starting point has been so often raised of late.
Among the assemblies which have occupied themselves with this question, the one which principally calls for our attention is that which was held at Rome last year; indeed, for many of our colleagues the conclusions adopted by the Congress of Rome settle the whole matter. These conclusions must, therefore, receive our special attention.