Page:Popular Science Monthly Volume 15.djvu/168



VERY one knows that what is called a first meridian is the circle from which we start in reckoning longitudes. It were better to call it an initial meridian, or zero meridian, for the first meridian is not in reality this one, but the first we meet in longitude starting from zero, i. e., at sixty minutes from this starting-point. We should prefer to adopt the term mediator, as proposed by M. Bouthillier de Beaumont, it being analogous to the term equator, which is the starting-point in reckoning latitudes.

This matter of a first meridian gives rise to very grave complications. Each nation wants to have its own meridian passing through its capital city, or its principal observatory. Hence result numerous difficulties, errors, even dangers and accidents to ships, in case we are not sure about the meridian employed, or if we are in error in our reckoning of the difference between one meridian and another.

The geographical knowledge of the ancients extended on the west only as far as the Canary Islands. From here, or hereabout, Ptolemy started in reckoning longitudes, going eastward to the limit of the countries then known. This western limit of his geographical knowledge he reckoned to be 60° west of Alexandria—a calculation which would place the starting-point a little to the west of the Canaries. According to Ptolemy's geography, Paris is in longitude 23°, and hence the starting-point could not be the most westerly isle of the Canary group, as has usually been supposed, but farther to the west. Nevertheless, to do away with all uncertainty, an ordinance of Louis XIII., in 1634, declared that French geographers must start from the isle of Ferro. But what was the precise situation of this isle? It was at first held to be 23° from Paris, and this erroneous calculation has given rise to strange variations in the position of the first meridian in a great number of maps.

In 1682 the observations of Varin and of Deshayes gave the longitude of Ferro as 20° 5′ west of Paris, and thenceforward the round number of 20° was taken to be the distance between these two meridians. Still many geographers, among them Delisle himself, who had been one of the first to make known the precise longitude of Ferro, continued to reckon the distance at 23°, after Ptolemy. Sometimes they corrected this reckoning, reducing it to 22°, or even to 20°. In 1711, in a map of the Île de France (Mauritius), Delisle places Paris in longitude 20° exactly, but the same geographer by a very strange anomaly, in a map bearing date 1717, adopts the figure 22° 30′. In fact, it