156 THE POPULAR SCIENCE MONTHLY. 



SELECTING A FIKST MERIDIAN.* 



By E. CORTAMBEET. 



EVERY one knows that what is called & 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 complica- 

 tions. 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 knowm. 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 longi- 

 tude of Ferro as 20 5' west of Paris, and' thenceforward the round 

 number of 20 was taken to be the distance between these two merid- 

 ians. 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 lie de France (Mauritius), Delisle places Paris in 

 longitude 20 exactly, but the same geographer by a very strange anom- 

 aly, in a map bearing date 1717, adopts the figure 22 30'. In fact, it 



* Translated from " La Nature," by J. Fitzgerald, A. M. 



