34 VHILOSOPHICAL TRANSACTIONS. [aNNO 1791' 



been so much affected by disease as to be uuable to restore themselves to a 

 natural state. 



VII. Cunsideralions on the Convenience of Measuring an Arch of the Meridian, 

 and of the Parallel of Longitude, having the Observatory of Geneva for their 

 Common Intersection. By Mark Augustus Pichtet, Professor of Philosophy 

 in the Academy of Geneva, p. 100. 



The accurate knowledge of the dimensions and true figure of the earth is not 

 a matter of mere curiosity. Astronomy and navigation are so closely connected 

 with it, that the philosophers of the present century have pursued this inquiry 

 through the most discouraging difficulties; and governments themselves have 

 contributed considerable sums towards its success. Notwithstanding these efforts, 

 the end is not yet obtained. There are 5 different conclusions on this subject; 

 one of which is given by Sir Isaac Newton's theory; the others are the result of 

 4 different measurements, which appear the most creditable among those that 

 have been performed. The extremes give -^^ and -j-i-r for the difference be- 

 tween the polar and equatorial diameters of the earth, that is, two fractions, one 

 of which is more than double the other. The cause of these disagreements is 

 yet unknown; perhaps the figure of the earth is really irregular; perhaps the se- 

 veral measurements have not been executed with the very minute exactness re- 

 quisite in so nice and so important an undertaking. 



The libera! and well-conducted operations carried on by the r. s., under the 

 direction of the late general Roy, for the trigonometrical determination of the 

 distance between the Observatories of Greenwich and Paris, render this last sup- 

 position extremely probable. It now seems evident, that the substances em- 

 ployed before for the actual measurement of the bases must have been influenced 

 in their length by pyrometrical and hygrometrical effects, which were either un- 

 known or ill-estimated at that time. The instruments also for observing the 

 celestial and terrestrial angles were far from the perfection to which they have 

 since been brought. In a word, the whole of the work should be again under- 

 taken with the far greater degree of accuracy which is now within our reach. 



Struck with the importance of these facts, Mr. P. transmitted, for the con- 

 sideration of the R. s., the present plan for measuring, by a commission of its 

 members, an arch of the meridian, and of a parallel of longitude, having the 

 Observatory of Geneva for their common point of intersection. Frequent ex- 

 cursions in the neighbouring mountains had convinced him, not only that the 

 n)easurement could be made, but that it would be perhaps the most easily exe- 

 cuted of any hitherto attempted. The best maps place the town of St. Jean da 

 Maurienne nearly south of Geneva, at the distance of about 38' of latitude. It 



