36 PHILOSOPHICAL TRANSACTIONS. [aNNO 1791. 



[jrotracted southwards is not intercepted by the mountains but at a great distance, 

 and in a place which must be near the southern end of the arch. If that place 

 should happen to be accessible, it would be a fortunate circumstance, as it would 

 offer a very simple, quick, and accurate verification of the direction of the me- 

 ridian line resulting from the chain of triangles, by actually protracting the visual 

 line given immediately by the transit instrument of the observatory down to the 

 end of the arch, by the help of 2 intermediate stations only. 



We see hitherto no local difficulties in the measurement of an arch of about 

 84' of the meridian of Geneva. The measurement of the parallel of lono-itude, 

 eastwards of the same place, seems to be of a still easier execution, as there are 

 few places on earth better disposed for the operation. The Republic of Vallais 

 in Switzerland offers an extensive, broad, and nearly straight valley, bordered on 

 both sides by high mountains. It is situated about the parallel of Geneva, runs 

 eastward for many leagues from the town of Martigny, and to the westward is 

 separated from the mountains of Chablais in Savoy, by a very lofty chain, in 

 which there is an accessible summit, called The Glacier de Biiet, or La Mortine. 

 This mountain is placed, as by a miracle, in such a position as to be visible from 

 the Observatory of Geneva, and about 10' west of it, as also from almost every 

 elevated situation in the Haut Vallais. Its summit is accessible by a much easier 

 ascent than that which was discovered by Mess, de Luc; and a signal made there 

 by the Indian lights would be visible east and west along the parallel to the whole 

 distance of perhaps 2° between the 2 extreme stations; for as the observations re- 

 lative to the regulating of the clocks do not require any considerable apparatus, 

 they could be performed in the most distant hamlets from which the signal should 

 be visible. 



As to the trigonometrical measurement along the parallel, it appears that it 

 might be executed with a smaller number of operations than that of the meridian 

 arch. Should the method, proposed by the late general Roy, Philos. Trans. 1787 

 for ascertaining the length of the parallel independently of astronomical obser- 

 vations, be adopted, it might be carried into execution with no great difficulty 

 from the summit of the same mountain, where we just now supposed the signal 

 by the Indian lights to be placed. The triangles relative to the measurement of 

 the parallel make but one suite with those of the meridian, and there are 4 very 

 convenient places along the same parallel for measuring bases of verification. 

 They are perfectly level plains, forming the bottom of the valley through which 

 the Rhone flows between the towns of Aigle and Villeneuve, and between Mar- 

 tigny and Sion. The above general considerations, together with the particulars 

 which are subjoined to the sketch, seem, Mr. P. thinks, to ascertain the full 

 practicability of the enterprize. He adds a few reflections on its convenicncy. 



