VOL. XL.] PHILOSOPHICAL TRANSACTIONS. ]"27 



In 1720, M. Mairan attempted to reconcile the two different hypotheses of 

 Sir Isaac and M. Cassini, by imagining that the earth, at its creation, being 

 without motion, was of a much more oblong figure than that which Cassini 

 thinks it has at present ; so that it might have been reduced to that which it 

 now has, by the diurnal motion on its axis, &c. But Dr. Desaguliers, who is 

 of Sir Isaac's opinion, has made appear, in the Philos. Trans. N" 388, that M. 

 Mairan's supposition is contrary to the laws of motion ; and has moreover pro- 

 posed several considerable doubts on the observations and suppositions employed 

 by M. Cassini, in his determination of the earth's figure in 17 18. 



As soon as the meridian of Paris had been extended from one sea to the 

 other, and M. Cassini had thence deduced a confirmation of the system of the 

 earth's being longer at the poles ; M. De Lisle imagined a new method of de- 

 ciding the question, viz. by the observation of the degrees of the parallel, com- 

 pared with those of the meridian. For that purpose he considered, that as the 

 degrees of the meridian and those of the parallel, at the same elevation of the 

 pole, had different relations, according to the different figures ascribed to the 

 earth ; nothing more was requisite for concluding which hypothesis was the 

 true one, than to determine this relation by immediate observation. 



Having supposed, that there had been observed on the parallel of Paris, a 

 space nearly of the saine magnitude with that on the meridian, that is, of about 

 13 degrees, since that on the meridian is about 84^ degrees ; he found by an 

 exact calculation, that according to the figure which M. Cassini has given to 

 the earth, this space ought to contain 13^ of the parallel, more than in the 

 hypothesis of the earth's being spherical ; which appeared considerable enough 

 to decide between these two hypotheses, and by a stronger reason between the 

 hypotheses of Newton and Cassini, seeing the difference ought to be still more 

 considerable than that now specified. 



He concluded, at least, that, independent of the figure of the whole earth 

 which could not be determined by the sole observations made in France, with- 

 out making suppositions, and admitting principles, which are still liable to be 

 contested ; it would be of great consequence towards constructing exact charts 

 of the kingdom, to ascertain this relation by observations, which consisted only 

 in forming triangles along the parallel of Paris, and observing at the two ends 

 the difference of the meridians, by the most exact methods. This difference 

 seemed to be so considerable, that he was in hopes of being able to determine 

 it by means only of two places within sight of each other, and situated to the east 

 and west; provided their difference of longitude were accurately observed, in- 

 dependently of astronomical observations, by means of lighted fires ; after the 

 manner that M. Picart put in practice in Denmark, for determining the difl^er- 



