VOL. LXXXI.^ PHILOSOPHICAL TRANSACTIONS. 35 



would be impossible to extend the measurement farther southwards, the central 

 and inaccessible chain of the Alps being in the way ; but if a greater arch should 

 be desired, it might be easily protracted about 26' north of Geneva. He thinks 

 that, the place being surrounded by mountains of nearly equal masses, and situa- 

 ted at almost equal distances, their effects would be hardly perceptible; and sup- 

 posing there should remain any doubt about their influence, this might be easily 

 ascertained by zenith distances, observed at the 2 extremities of a little plain in 

 which the town is built, and compared with the real distance of the stations, 

 determined by an actual measurement. That town being the residence of a 

 bishop, and containing near 3000 inhabitants, might furnish the observers with 

 a convenient building for the zenith sector, and the occasional help and neces- 

 saries which might be required. The great post-road from hence into Italy, over 

 Mount Cenis, passing through it, is also an advantageous circumstance. 



The disposition and bearing of the valleys from that town, which would be 

 the southernmost extremity of the arch, is advantageous for the series of triangles: 

 for Mr. P. has seen from the top of a mountain near St. Jean, called Le Mont 

 Sapey, 2 parallel chains extending to the north on both sides of the river Arc, 

 and there appeared to be in their summits a great choice for convenient stations, 

 as far as the confluence of the Arc and the river Isere near Aiguebelle, whence 

 the mountains in the parallel of Chamberi are all visible. From this last parallel 

 to Geneva, and farther, there are not only no difiiculties, but the stations are 

 for the greatest part already determined. The visible part of the meridian of 

 Geneva is soon terminated northwards by the first chain of Mount Jura; but 

 the country opens to the n.n. e. and the northern. station might be easily chosen 

 in some place of the Pays de Vaud, visible from the Observatory of Geneva, and 

 which could be determined by only 1 additional triangle. He points out 2 such 

 places. The one, called Vincy, about l6' north of Geneva. The other place 

 is the top of a mountain, called the Dent de Vaulion, making part of the chain 

 of Mount Jura, and where an occasional observatory might be erected without 

 much difficulty: it is 10' north of Vincy, or 26' of Geneva. The whole arch 

 from St. Jean de Maurienne to this last place would be about 1° 24'. The ce- 

 lestial observations might perhaps be made in the 4 places above mentioned; and 

 the meridian arch would be thus obtained in 3 portions, whose comparison with 

 the terrestrial sections, measured geometrically, would be a proof of the accuracy 

 of the operation. 



The southern part of the meridian line, visible from the Observatory of 

 Geneva, passes over the summit of a mountain called Mount Saleve, where they 

 have a meridian mark, at the distance of about 5600 toises, and at the height of 

 about 500 toises above the level of the lake. From that summit, the same line 



F 2 



