132 MAPS AND THEIR MAKERS 



forms of any considerable area thus appear curiously un- 

 integrated. It must be remembered, however, that it was not 

 for many years that sufficient determinations of altitude existed 

 for the relief to be represented with any accuracy. 



Whatever the defects of the map it is a remarkable monu- 

 ment in the history of cartography, and one which influenced 

 the mapping of many countries. It was not for half a century 

 after its initiation that a comparable undertaking was begun 

 in Britain by the Ordnance Survey. It was incidentally on 

 the initiative of Cassini de Thury that General Roy was com- 

 missioned to co-operate in the cross-Channel triangulation of 

 1787, and in so doing paved the way for the foundation of the 

 Ordnance Survey. 



Having traced the history of the Cassini survey, we may 

 now examine the progress of general cartography in France. 

 J. D. Cassini's method of determining longitude was employed 

 at an early date in fixing positions outside France. Largely 

 with a view to improving existing marine charts, observers 

 from the last decades of the seventeenth century onwards 

 were sent to various countries in Europe, French Guiana, the 

 West Indies, Africa, and southern and eastern Asia, where in 

 course of time remarkably accurate values were obtained. 

 From Richer's observations, for example, the longitude of 

 Cayenne was determined within one degree of its true value. 

 The earlier results enabled J. D. Cassini in 1682 to sketch out 

 his famous planisphere, which incorporated forty determina- 

 tions, on a floor of the Paris observatory. This was later 

 engraved with the title Tlanispherum terrestre', an issue of 

 which is known from 1694. These new observations were also 

 the basis of a collection of sea charts covering, on Mercator's 

 projection, the western coasts of Europe from Norway to 

 Spain; this was 'Le Neptune frangois, ou Atlas nouveau des 

 cartes marines. . . . Revue et mis en ordre par les Sieurs Pene, 

 Cassini et autres', Paris, 1693. 



The man who introduced this new work to the general 

 public, and in doing so effected what has been called the 

 ^reformation of cartography', was Guillaume Delisle (1675- 

 1726). Guillaume was the son of Claude Delisle, a celebrated 

 teacher in his day of history and geography, to whom un- 



