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.