104 PHILOSOPHICAL TRANSACTIONS. [aNNO 1675. 



search and clearing of this difficulty. That for this purpose, Alniamon, an 

 Arabian prince, ordered experiments to be made in the plains of Sanjar; where 

 a station being chosen, thence two troops of horsemen set out, and pro- 

 ceeded opposite ways, in a straight line, till one of them had raised a degree of 

 latitude, and the other had depressed it ; at the end of their marches, they who 

 raised it, counted 56^ miles, and they who depressed it reckoned just 56 miles. 

 From this observation can be known but very little, because we know not of 

 what length these miles were. He further observes, that the ancient compu- 

 tations of miles for a degree ran always on the decrease; thus, Aristotle count- 

 ing to a degree 1111 stadia : after him Eratosthenes counted but 700 ; Possi- 

 donius only 666; Ptolemy but 500. Besides, the precise length of these 

 stadia is unknown, and that they were also different among themselves; the 

 stadia of Alexandria differing from those of Greece. At last Fernelius brought 

 it to 56,746 toises or fathoms of Paris, each of which is equal to 6 Parisian feet; 

 Snellius to 55,021. The method of this last M. Picard judges to be the most 

 ingenious ; which was by a scale or series of triangles. But in one thing he 

 esteems it deficient, which is, that Snellius took his objects only by common 

 sights, which do not so distinctly point them out. 



M. Picard, in his measuring of a degree, chose a meridian, out of which he 

 intended to take his measure, between Sourdon in Picardy, and Malvoisin, on 

 the confines of Gastinois and Hurepois ; and he actually measured a way that 

 lay very straight, between Villejuifve and Ivoisy, viz. the line AB, (fig. 7, pi. 7); 

 and he found the distance between these two terms, in going forward, to be 



5662 toises and 5 feet, and in coming back 5663 toises and 1 foot ; which being 

 measured with great exactness, he stated the distance between these two places, 

 in round numbers at 5663 toises. The intruments he measured with, were 

 pikes joined together at their ends by a screw, which measure was 4 toises lojig: 

 This he applied along a cord stretched horizontally, and at the end of every 

 such pike placed a stake, of which stakes he had 10 in all. This distance of 



5663 toises was the base of the first triangle, on which the measure of all the 

 depending series was formed. The instrument for taking the angles was a 

 quadrant of 38 inches radius, furnished with telescopic sights. He takes occa- 

 sion, by the bye, to speak of measures in general ; and says, that a pendulum 

 vibrating a second of time, computed according to the mean motion of the sun, 

 is 36 inches and 8 ^ lines of the Paris measure. And he thinks that this mea- 

 sure may probably serve in all countries, because the same length of a pendulum 

 served for a second both at the Hague and at Paris; whence he conjectures, 

 the same may serve also in other latitudes. Hence he infers, that if it were 

 desired to constitute a universal measure, which might be common to all coun- 

 tries^ it might be thus made^ viz. Call this pendulum for seconds, of 36 inches 



