The construction of the meridian line of the Paris Observatory, which had been initiated by his father, was continued by Jacques Cassini, called Cassini II, and fully completed in 1732. The main purpose of this meridian line was to measure the variation of the obliquity of the ecliptic. The object of this article is to study this measure, by carrying out an unprecedented examination of part of the manuscript registers of the Paris Observatory, which include all observations made until the end of the 18th century. The twenty-five years of measures (1730-1755) that will concern us here have not been published before. They represent a wealth of material as yet unexploited, which enables us to grasp more fully the metrological and historical aspects of the meridian line of the Paris Observatory.